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Samuel Pierpont Langley 1834–1906. The solar spectrum and the solar constant
Solar & Wind Technology Vol. 5, No. 1, pp. 93-97, 1988 Printed in Great Britain.
0741~83X/88 $3.00+.00 Pergamon Journals Ltd.
SAMUEL PIERPONT LANGLEY 1834-1906. THE SOLAR SPECTRUM A N D THE SOLAR CONSTANT L. B. MULLETT Department of Engineering, University of Reading, Whiteknights, Reading, U.K.
(Received20 February 1987; accepted 16 July 1987) AImtract--4)nly very recently has it been observed that the still accepted values of absorption of solar radiation in water were based on the original Langley spectrum as published in the report of his expedition to Mount Whitney in 1881. As compared with a modem spectrum there is a marked deficiencyin deeply penetrating violet and ultraviolet radiation. However, the spectrum persistently published by Langley in the scientificliterature and reproduced since, is still worse in this respect owing to an error in the material first given to the engraver. Also, throughout his life, Langley believedthe solar constant to be 3 Langleys, again based on measurement made on Mount Whitney. Yet had he not misinterpreted the data he would have obtained a value very close to the presently accepted figure of 1.94 Langleys. INTRODUCTION We are indebted to Langley for the first mapping in detail of the solar spectrum much as we know it today, particularly in the infra-red region which he extended to 2.8/~ and beyond from the former measured limit around 1.0/~. It was to this ever more detailed task that his principal attention was directed for many years. The determination of the solar constant, though a matter of great concern at the outset, seems later to have received the lesser part of the resources available to him. It is far less widely known, however, that between 1891 and 1896, a great deal of his time and effort was taken by experiments in aerodynamics using a whirling table, and the development of flying machines (for which, confusingly, he devised the term "aerodrome"--meaning "air runner"). In 1895, his sixth steam-driven model was successfully launched by catapult from a float on the Potomac river and "flew" for almost a mile, but his full scale craft, now with the benefit of an internal combustion engine, twice met a sad fate. It is still less well known that in 1898 he was allocated US army funds to build a flying machine large enough to carry a man, but again, in 1903, launch on the Potomac river was a disappointing failure [1].
TIlE SOLAR SPECTRUM To return to solar energy, Langley was convinced of the need for a new detector, so sensitive as not only to extend the then known range of the spectrum but 93
to be able to measure it throughout with such accuracy for different air masses so as to obtain the absorption coefficient of the atmosphere as a function of wavelength. Only in this way would he contemplate obtaining valid extrapolation to the solar constant. To this end he invented the bolometer, a strip of metal foil so fine and with so sensitive a galvometer (in a bridge arrangement) as to measure currents of the order of say 6 x 10-1~ A, equivalent to 3 x 10 -5 °C. His "air mass 1" spectrum was obtained from measurements made at the same height but at different times at the Allegheny Observatory of the University of Pennsylvania in 1882/1883. This he first published in the American Journal of Science in March 1883 [2], and continued throughout his life to publish in exactly the same form, e.g. in 1896 to the French Academy of Science [3]. It is this spectrum which authors, as late as Robinson in 1966 [4], frequently published as the solar spectrum, often without proper attribution. It is readily recognized by having beneath it a chart of absorption lines. Yet the Allegheny spectrum reproduced in 1884 in his report of the Mount Whitney expedition [5] is distinctly different, having its peak further towards the violet end with more violet and ultraviolet radiation. The mystery deepens in that the first accurate calculation of the absorption of solar radiation in water (especially for oceanography) was published by Schmidt in 1903 [6] using an unspecified spectrum which turns out to be that used in the report of the expedition to Mount Whitney. Tsilingiris [7] noted, only as recently as 1985, that this spectrum was much different from a modern air-mass 1 spectrum, with its peak at 0.6 p rather than somewhat below 0.5 /~, and with
94
L. B. MULLETT modern"air massI"
t~t0y- correct
/
o~
.::<
0'5 0-6 0"? 0"6 0.9 1'0 1'1
.
1.2 1.] 1.t+ 1"5 1.6 1.7 1'8 1"9 2"0 2-1 2"2 2'3 2.t+ 2"5 n~crons Fig. 1. Normal spectrum.
much less violet and ultraviolet radiation to which water is so transparent. So much so, that Schmidt's calculations, though beautifully and accurately done at the time, very much underestimate the solar penetration. As a result, the collection efficiency of salt gradient solar ponds has been much underestimated, and no doubt much oceanographic data are highly questionable, since both were, and often still are, based on a table of data going right back to those early days. The situation would have been still worse had Schmidt used the version of the spectrum which Langley himself consistently published in the scientific literature and which everyone else copied. Who and what was right? One can deduce from the prismatic spectrum, that the version of the normal spectrum given in the Mount Whitney report was correct, so that Schmidt was right, and sure enough one finds in the Mount Whitney report a footnote to p. 134 : The maximum ordinate on the normal curve, as deduced from the prismatic one, should fall in the same place as that observed directly in the heat spectrum of a grating, and actually does so in our charts with gratifying exactness. In the illustration first given in the American Journal of Science and other journals, owing to a slight error in the preparation of the drawing for the engraver, the maximum ordinate of the normal curve is somewhat nearer the red than it should be, and the descent of the curve towards the violet end somewhat less abrupt. The three versions of the spectrum are reproduced in Fig. 1 The incorrect Langley spectrum shows the infra-red absorption bands (which, as Langley and his people spent some 10 years measuring, are the summation of very large numbers of very narrow absorption lines) together with a smoothed line
(shown as dotted). The correct Langley spectrum, which, to our knowledge, was used only by Schmidt, is shown in the smoothed form and is "normalized" to the same area--that is, as if each were correctly measured in total as by, say, a calorimeter. The modern air-mass 1 spectrum is again smoothed in the infrared region and roughly normalized. Anyone performing an accurate calculation should use the spectrum in tabular form and equate the integrated area to the total measured radiation. THE SOLAR CONSTANT Throughout his life, Langley strongly persisted in the view that could he mount an expedition to a sufficiently high altitude he would find a value for the solar constant of at least 3 Langley's (cal. min.cm-2). The currently accepted figure in these units is 1.94 (or 1353 W m-2). Langley believed, correctly of course, that since atmospheric attenuation was a function of wavelength, it was necessary first to make such measurements, then to extrapolate and finally to integrate. But, by some dubious reasoning, he deduced that the solar constant measured by a pyrheliometer (that is, ealorimetrically) for all wavelengths together and extrapolated by Bouguer's law, would be far too low. The original measurement by Pouillet with a water calorimeter was 1.7 Langleys, and later workers, Soret, Crova and Violle, placing more and more reliance on indirect actinometers, produced a succession of figures up to 2.5. Believing also, and again correctly, that the dirtiest most absorbing part of the atmosphere was the lowest, Langley formed his belief of a figure of 3.0 or thereabouts.
Langley 1834--1906 It was with all this in mind and with the spectrobolometer barely invented, that through the generosity of the people of Pittsburgh and the help of the US Secretary of War, he undertook an expedition to Mount Whitney in the Sierra Nevada range in southern California (latitude 36°36'). There was no railway at that time, indeed it was not long after the Indian Wars, so that the expedition had to be mounted (literally) by the US Signal Service. Base camp was set up at Lone Pine (a notable feature of which, was the cemetery) at a height of 1184 m and mountain camp at 3581 m. Pyrheliometer, actinometer and spectrobolometer measurements were made at both camps and pyrheliometer measurements were even made at the peak of about 4500 m. The logistics alone were an outstanding feat in those days and to take delicate scientific measurements with very sensitive equipment, with only tents for shelter, heat, cold and sand in everything, ranks with the best in scientific exploration. The pyrheliometers were water calorimeters essentially as used by Pouillet, but corrected for poor conduction of the water by comparison with a mercury pyrheliometer back at Allegheny. At Lone Pine, and for clear sky conditions, the mean value of the solar constant obtained from high and low sun was 1.760--remarkably close to Pouillet's 1.764. Langley's comment was "The only significance of this appears to be that like methods bring about like results". Both numbers are, as we now know, significantly low. Yet the mean transparency of the atmosphere at Lone Pine was 87% giving an actual insolation value of 1.53 Langleys, as compared with 80% near sea-level at Allegheny and 1.41 Langleys, much as we know them now.
However, Langley put greater faith in the actinometers of Violle and Crova, essentially measuring the rise and fall in temperature of the bulb of a mercury thermometer exposed to the rays of the sun at the centre of a water (or ice) cooled sphere to which the bulb radiates. On the expedition, Langley used a large globe provided by M. Voille, and two smaller "home made" versions. The relationships of heating and cooling to the magnitude of the sun's rays was obtained by calculation, rather than calibration with a pyrheliometer. At Lone Pine, the midday value of the total insolation thus obtained was in the region of 1.77 Langley's, and from high and low sun the extrapolated value of the solar constant was 2.019 Langleys. At Mountain Camp it was 2.131 Langleys. Although a value of 2.02 Langleys was remarkably close to what we now know the solar constant to be (1.94 Langleys), Langley expected a further, indeed
95
a major increase if the variation of absorption with wavelength could be obtained with the spectrobolometer. The majority of the spectrobolometer measurements were made with a diffraction grating having a wavelength limit of 1.2 # (the extent of the then known spectrum). Towards the end, a prism was used and a great extention of the spectrum was found to 2.7 #. Once found, this region was fully explored back at Allegheny. This then is the well known Langley spectrum, subject to the confusion already explained. F o r the purpose of determining transmission coefficients at Lone Pine and Mountain Camp, the spectra measured there were augmented to 2.4 # with Allegheny data. The area of the bolometer curve had to be normalized to the measurement of total radiation. It was the actinometer measurements which were used. The resulting coefficients of transmission as functions of wavelength were obtained for the air between Lone Pine and Mountain Camp, and above Mountain Camp. The former, weight for weight, are very much lower, particularly and progressively for wavelengths below 1.0 #. As far as was possible within the limitations of the data, a two stage calculation was made of the spectrum outside the atmosphere and a value of 3.5 Langleys was obtained for the solar constant. Langley described this as a "maximum" value. Later, by various "manipulations" he obtained a "best value" of 3.07 Langleys. In 1886, Langley became Secretary of the Smithsonian Institute in Washington and followed the tradition for the Secretary to continue his researches. He set up the Astrophysical Observatory in 1892 and remained Director of it until his death in 1906. The majority of the work done until the turn of the century concerned the development and perfection of the spectrobolometer, and the automated mapping of the enormous number of absorption lines in the apparent absorption bands. It was not until the new century that attention turned again to the spectrum as a means of obtaining the coefficient of air transmission as a function of wavelength, so as to determine the solar constant to the best of ability at that time. C. G. Abbot, then Aid to the Director, commented in the Annual Report of the Institution for 1902 [8] on the need to determine the absorption of the spectrometer itself to obtain the true distribution. "As the mirror surfaces deteriorate, this has to be done not infrequently". Clearly silvered mirrors give reduced reflection without apparent deterioration, especially at the shorter (violet and ultra-violet) wavelengths. In the Annual Report for 1903 [9], he reported that it was only since January of that year that the region of 0.375~).47 could be included in regular bolographic work. (By
96
L. B. MULLETT
then the spectrobolometer had been automated to draw the spectra directly.) What is more, higher atmospheric attenuation was now being found in this region, and a computed spectrum for outside the atmosphere peaked at 0.48 #, much as we now find it. Attention turned again to determining the solar constant, and one must refer increasingly to the work of Abbot and Fowle (a "Junior Assistant"). The progress of the work is "plotted" in the annual reports of the Smithsonian Institution, where there was always a section on the Astrophysical Observatory, written by the Aid Acting in Charge (Abbot) to Langley as Director of the Laboratory and Secretary of the Institution. The first six months of both 1900 and 1901 were taken up with expeditions in connection with solar eclipses. However, by 1902, further improvements had been made in the speedy and accurate plotting of solar spectra (so avoiding problems due to changing sky conditions) and in the computation of transmission as a function of wavelength. In 1903, with these new spectra and with measurements of total radiation using a mercury pyrheliometer and a Crova alcohol actinometer, values of the solar constant were obtained from high and low sun data in Washington. The mean of results before March 1903 was 2.229 Langleys, and after that date 2.080. There was much speculation as to whether there had been a lessening of solar radiation. Nevertheless, in a paper to the Astrophysical Journal in March 1903 [10], Langley was unrepentant giving a "provisional" value of 2.54 and a firm opinion that measurements near sea-level would always give a smaller result than at elevation, quoting again his Mount Whitney results and pressing the need for an elevated solar laboratory. By June 1904 in a paper to the Astrophysical Journal [11] he gave the 1902/1903/1904 results for the solar constant as ranging from 1.93 to 2.29, but expressed doubt as to the absolute validity of the actinometry to within 20% and again pressed for an elevated station. This time the funds were forthcoming and in May 1905 Abbot and his team left for Mount Wilson with all the necessary equipment. The average value of the solar constant from measurements made at a height of 1780 m by October of that year was 2.024 Langleys. By then, Langley, though mentally active, was seriously ill and died in February 1906. He was almost certainly aware of the results, but we do not know whether he appreciated their significance. Abbot, however, thought he did. Earlier in 1903 he had reported to Langley "All the results depend upon the constants of the pyrheliometer and may therefore be subject to multiplication by a constant factor to be subsequently determined. Comparing the values
which you have given in the Mount Whitney report of 3 calories, it will be seen that they are 25% smaller, and the difference does not appear to depend on the transmission coefficient but rather seems chiefly due to a difference in actinometry. Thus you have stated that the usual actinometer reading at Allegheny, Pa., for clear blue sky at 1.7 calories, while the very highest value obtained here is 1.44 calories". But still, the fact is stranger than fiction; though it might read like it. As we have already seen, Langley first obtained a value of 3.5 Langleys from his transmission measurements on Mount Whitney and chose to call this his maximum value. He then proceeded to determine a "minimum" value, using high and low sun values of the transmission coefficients. The measurements so obtained at Lone Pine related to the whole atmosphere above Lone Pine. Langley quotes an example of a reading of 201 (unspecified units) from the bolometer at a wavelength of 0.6 p, the peak of the spectrum. Using the high and low sun transmission coefficient, the computed value for Mountain Camp was 206. However, the actual Mountain Camp reading was 249.7. Similar high and low sun observations at Mountain Camp gave 275 outside the atmosphere. Langley then applied a correction of 249.7/206, arguing that if one had been able to ascend above the atmosphere there would have been the same shortfall as at Mountain Camp from the Lone Pine calculation. The same procedure was followed at all wavelengths, giving a solar constant of 2.63 Langleys. Langley then "devised" an intermediate spectrum from which he derived his "best" value of 3.07 Langleys. There may, or may not have been a systematic error, or even a real effect in the Lone Pine/Mountain Camp data (due to the bolometer, the actinometer, the weather or the fact that the lower atmosphere up to Mountain Camp was more absorbant than the total atmosphere above Lone Pine) but the Mountain Camp high and low sun data, being relevant only to the atmosphere above the camp, should stand entirely on their own benefit. Taking the extrapolated peak of the botometer distribution outside the atmosphere as 275, it can be seen from the Langley spectrum that the solar constant would have been 2.3 Langleys. In fact, Langley tabulates solar constant values obtained in this way which average 2.22 as compared with 2.06 from high and low sun values at Lone Pine. The Mountain Camp figure of 2.22 should be taken to be the more realistic since there was less atmosphere which to be uncertain about. Had Langley then believed the absolute pyrheliometers rather than the indirect actinometers, he would have obtained a value
Langley 1834-1906 of 1.92, remarkably close to the presently accepted value of 1.94. In 1906, after Langley's death, an unidentified translator of a paper by R a d m a n on " A s t r o n o m y on M o n t Blanc" reproduced in the Smithsonian Report [12], added a footnote: Langley's value, 3 calories, depends on an inference which he made in 1881 of a failure of Bouguer's formula for allowing for atmospheric absorption, even when applied to homogenous rays. Nevertheless he made no correction in his later papers for t h i s . . . That this was most likely A b b o t can be deduced from the Annual Report for 1907 [13] where, now as Director of the Astrophysical Observatory, he wrote: The results of Langley, while seemingly in contradiction of these (results), in reality support them. For as he states at page 211 of the Report of the Mount Whitney expedition, his value (3 calories) for the "solar constant" depends upon an allowance which he made for an apparent "systematic error in high and low sun observations at one station", of such a nature as becomes manifest "by calculating at the lower station, from our high and low sun observations there, the heat which should be found at a certain height in the atmosphere, then actually ascending to this height, and finding the observed heat there conspicuously and systematically greater than the calculated one". In fact there is no such systematic error, no correction for it should have been applied by Langley, and the best mean value of his experimental determination of the "solar constant" at Mount Whitney and Lone Pine is 2.114 calories per square centimeter per minute. Finally, after a more "respectable" time interval, A b b o t wrote in the second volume of the Annals o f the Astrophysical Observatory [14]: Owing to this most unfortunate error, the great authority of Langley has supported the value of 3.0 calories for the last twenty five years, and many observers have been perhaps wrongly influenced by it. In reality, Langley's Mount Whitney observations support the value 2.1 calories. Indeed, as the last word, and with our still longer hindsight, Langley's value of the solar constant could well have been precisely 1.92 calories.
97 CONCLUSION
F o r our older scientists, there is clearly much o f research as well as personal interest to be found in the archives. F o r our younger scientists there is the moral that even though it may take another I00 years, their transgressions against scrupulous accuracy and open mindedness may still be found out. REFERENCES
1. C. Alder, Samuel Pierport Langley, Annual Report of the Board of Regents of the Smithsonian Institute for the year ending 30 June 1906. 2. S.P. Langley, The Selective Absorption of Solar Energy, Am. J. Sci. Third Series, 169-196 (1883). 3. S. P. Langley, C. R. Seances L',4cad. Sci. 95, 482-487 (1896). 4. N. Robinson, Solar Radiation. Elsevier (1966). 5. S.P. Langley, Researches on solar heat and its absorption by the earth's atmosphere. A report of the Mount Whitney Expedition (1881), Professional Peapers of the Signal Service No. XV, US War Dept., (1884). 6. W. Sehmidt, Absorption des Sonnenstrahling in Wasser, Sitzungsberichte des Mathematisch--Naturwissenschaftlichen Klasse, der Kaiserlichen Academie der Wissensehaften, CXVII Band, Abteihing IIa, Jahrgang 1908, Heft IBis X, pp. 237-253 (1908). 7. P. T. Tsilingiris, Analytical and experimental studies on Salt Gradient Solar Ponds. Ph.D. Thesis, University of Reading (1985). 8. C. G. Abbot, Report of the work of the Astrophysical Observatory, Annual Report of the Board of Regents of the Smithsonian Institution, for the year ending 30 June 1902. 9. C. G. Abbot, ibid., to 30 June 1903. 10. S. P. Langley, The solar constant and related problems, Astrophys. J. XVII, No. 2, March 1903. 11. S. P. Langley, On a possible variation of the solar radiation and its probable effect on terrestrial temperatures, Astrophys. J. XIX, No. 5, June 1904. 12. Unknown translator of a paper by H. Radau, Astronomy on Mont Blanc, Annual Report of the Board of Regents of the Smithsonian Institution, for the year ending 30 June 1906. 13. C. G. Abbot, Report of the work of the Astrophysical Observatory, Annual Report of the Board of Regents of the Smithsonian Institution for the year ending 30 June 1907. 14. C. G. Abbot, Annals of the Astrophysical Observatory of the Smithsonian Institution, Vol. 11, for the years 1900-1907.
0741~83X/88 $3.00+.00 Pergamon Journals Ltd.
SAMUEL PIERPONT LANGLEY 1834-1906. THE SOLAR SPECTRUM A N D THE SOLAR CONSTANT L. B. MULLETT Department of Engineering, University of Reading, Whiteknights, Reading, U.K.
(Received20 February 1987; accepted 16 July 1987) AImtract--4)nly very recently has it been observed that the still accepted values of absorption of solar radiation in water were based on the original Langley spectrum as published in the report of his expedition to Mount Whitney in 1881. As compared with a modem spectrum there is a marked deficiencyin deeply penetrating violet and ultraviolet radiation. However, the spectrum persistently published by Langley in the scientificliterature and reproduced since, is still worse in this respect owing to an error in the material first given to the engraver. Also, throughout his life, Langley believedthe solar constant to be 3 Langleys, again based on measurement made on Mount Whitney. Yet had he not misinterpreted the data he would have obtained a value very close to the presently accepted figure of 1.94 Langleys. INTRODUCTION We are indebted to Langley for the first mapping in detail of the solar spectrum much as we know it today, particularly in the infra-red region which he extended to 2.8/~ and beyond from the former measured limit around 1.0/~. It was to this ever more detailed task that his principal attention was directed for many years. The determination of the solar constant, though a matter of great concern at the outset, seems later to have received the lesser part of the resources available to him. It is far less widely known, however, that between 1891 and 1896, a great deal of his time and effort was taken by experiments in aerodynamics using a whirling table, and the development of flying machines (for which, confusingly, he devised the term "aerodrome"--meaning "air runner"). In 1895, his sixth steam-driven model was successfully launched by catapult from a float on the Potomac river and "flew" for almost a mile, but his full scale craft, now with the benefit of an internal combustion engine, twice met a sad fate. It is still less well known that in 1898 he was allocated US army funds to build a flying machine large enough to carry a man, but again, in 1903, launch on the Potomac river was a disappointing failure [1].
TIlE SOLAR SPECTRUM To return to solar energy, Langley was convinced of the need for a new detector, so sensitive as not only to extend the then known range of the spectrum but 93
to be able to measure it throughout with such accuracy for different air masses so as to obtain the absorption coefficient of the atmosphere as a function of wavelength. Only in this way would he contemplate obtaining valid extrapolation to the solar constant. To this end he invented the bolometer, a strip of metal foil so fine and with so sensitive a galvometer (in a bridge arrangement) as to measure currents of the order of say 6 x 10-1~ A, equivalent to 3 x 10 -5 °C. His "air mass 1" spectrum was obtained from measurements made at the same height but at different times at the Allegheny Observatory of the University of Pennsylvania in 1882/1883. This he first published in the American Journal of Science in March 1883 [2], and continued throughout his life to publish in exactly the same form, e.g. in 1896 to the French Academy of Science [3]. It is this spectrum which authors, as late as Robinson in 1966 [4], frequently published as the solar spectrum, often without proper attribution. It is readily recognized by having beneath it a chart of absorption lines. Yet the Allegheny spectrum reproduced in 1884 in his report of the Mount Whitney expedition [5] is distinctly different, having its peak further towards the violet end with more violet and ultraviolet radiation. The mystery deepens in that the first accurate calculation of the absorption of solar radiation in water (especially for oceanography) was published by Schmidt in 1903 [6] using an unspecified spectrum which turns out to be that used in the report of the expedition to Mount Whitney. Tsilingiris [7] noted, only as recently as 1985, that this spectrum was much different from a modern air-mass 1 spectrum, with its peak at 0.6 p rather than somewhat below 0.5 /~, and with
94
L. B. MULLETT modern"air massI"
t~t0y- correct
/
o~
.::<
0'5 0-6 0"? 0"6 0.9 1'0 1'1
.
1.2 1.] 1.t+ 1"5 1.6 1.7 1'8 1"9 2"0 2-1 2"2 2'3 2.t+ 2"5 n~crons Fig. 1. Normal spectrum.
much less violet and ultraviolet radiation to which water is so transparent. So much so, that Schmidt's calculations, though beautifully and accurately done at the time, very much underestimate the solar penetration. As a result, the collection efficiency of salt gradient solar ponds has been much underestimated, and no doubt much oceanographic data are highly questionable, since both were, and often still are, based on a table of data going right back to those early days. The situation would have been still worse had Schmidt used the version of the spectrum which Langley himself consistently published in the scientific literature and which everyone else copied. Who and what was right? One can deduce from the prismatic spectrum, that the version of the normal spectrum given in the Mount Whitney report was correct, so that Schmidt was right, and sure enough one finds in the Mount Whitney report a footnote to p. 134 : The maximum ordinate on the normal curve, as deduced from the prismatic one, should fall in the same place as that observed directly in the heat spectrum of a grating, and actually does so in our charts with gratifying exactness. In the illustration first given in the American Journal of Science and other journals, owing to a slight error in the preparation of the drawing for the engraver, the maximum ordinate of the normal curve is somewhat nearer the red than it should be, and the descent of the curve towards the violet end somewhat less abrupt. The three versions of the spectrum are reproduced in Fig. 1 The incorrect Langley spectrum shows the infra-red absorption bands (which, as Langley and his people spent some 10 years measuring, are the summation of very large numbers of very narrow absorption lines) together with a smoothed line
(shown as dotted). The correct Langley spectrum, which, to our knowledge, was used only by Schmidt, is shown in the smoothed form and is "normalized" to the same area--that is, as if each were correctly measured in total as by, say, a calorimeter. The modern air-mass 1 spectrum is again smoothed in the infrared region and roughly normalized. Anyone performing an accurate calculation should use the spectrum in tabular form and equate the integrated area to the total measured radiation. THE SOLAR CONSTANT Throughout his life, Langley strongly persisted in the view that could he mount an expedition to a sufficiently high altitude he would find a value for the solar constant of at least 3 Langley's (cal. min.cm-2). The currently accepted figure in these units is 1.94 (or 1353 W m-2). Langley believed, correctly of course, that since atmospheric attenuation was a function of wavelength, it was necessary first to make such measurements, then to extrapolate and finally to integrate. But, by some dubious reasoning, he deduced that the solar constant measured by a pyrheliometer (that is, ealorimetrically) for all wavelengths together and extrapolated by Bouguer's law, would be far too low. The original measurement by Pouillet with a water calorimeter was 1.7 Langleys, and later workers, Soret, Crova and Violle, placing more and more reliance on indirect actinometers, produced a succession of figures up to 2.5. Believing also, and again correctly, that the dirtiest most absorbing part of the atmosphere was the lowest, Langley formed his belief of a figure of 3.0 or thereabouts.
Langley 1834--1906 It was with all this in mind and with the spectrobolometer barely invented, that through the generosity of the people of Pittsburgh and the help of the US Secretary of War, he undertook an expedition to Mount Whitney in the Sierra Nevada range in southern California (latitude 36°36'). There was no railway at that time, indeed it was not long after the Indian Wars, so that the expedition had to be mounted (literally) by the US Signal Service. Base camp was set up at Lone Pine (a notable feature of which, was the cemetery) at a height of 1184 m and mountain camp at 3581 m. Pyrheliometer, actinometer and spectrobolometer measurements were made at both camps and pyrheliometer measurements were even made at the peak of about 4500 m. The logistics alone were an outstanding feat in those days and to take delicate scientific measurements with very sensitive equipment, with only tents for shelter, heat, cold and sand in everything, ranks with the best in scientific exploration. The pyrheliometers were water calorimeters essentially as used by Pouillet, but corrected for poor conduction of the water by comparison with a mercury pyrheliometer back at Allegheny. At Lone Pine, and for clear sky conditions, the mean value of the solar constant obtained from high and low sun was 1.760--remarkably close to Pouillet's 1.764. Langley's comment was "The only significance of this appears to be that like methods bring about like results". Both numbers are, as we now know, significantly low. Yet the mean transparency of the atmosphere at Lone Pine was 87% giving an actual insolation value of 1.53 Langleys, as compared with 80% near sea-level at Allegheny and 1.41 Langleys, much as we know them now.
However, Langley put greater faith in the actinometers of Violle and Crova, essentially measuring the rise and fall in temperature of the bulb of a mercury thermometer exposed to the rays of the sun at the centre of a water (or ice) cooled sphere to which the bulb radiates. On the expedition, Langley used a large globe provided by M. Voille, and two smaller "home made" versions. The relationships of heating and cooling to the magnitude of the sun's rays was obtained by calculation, rather than calibration with a pyrheliometer. At Lone Pine, the midday value of the total insolation thus obtained was in the region of 1.77 Langley's, and from high and low sun the extrapolated value of the solar constant was 2.019 Langleys. At Mountain Camp it was 2.131 Langleys. Although a value of 2.02 Langleys was remarkably close to what we now know the solar constant to be (1.94 Langleys), Langley expected a further, indeed
95
a major increase if the variation of absorption with wavelength could be obtained with the spectrobolometer. The majority of the spectrobolometer measurements were made with a diffraction grating having a wavelength limit of 1.2 # (the extent of the then known spectrum). Towards the end, a prism was used and a great extention of the spectrum was found to 2.7 #. Once found, this region was fully explored back at Allegheny. This then is the well known Langley spectrum, subject to the confusion already explained. F o r the purpose of determining transmission coefficients at Lone Pine and Mountain Camp, the spectra measured there were augmented to 2.4 # with Allegheny data. The area of the bolometer curve had to be normalized to the measurement of total radiation. It was the actinometer measurements which were used. The resulting coefficients of transmission as functions of wavelength were obtained for the air between Lone Pine and Mountain Camp, and above Mountain Camp. The former, weight for weight, are very much lower, particularly and progressively for wavelengths below 1.0 #. As far as was possible within the limitations of the data, a two stage calculation was made of the spectrum outside the atmosphere and a value of 3.5 Langleys was obtained for the solar constant. Langley described this as a "maximum" value. Later, by various "manipulations" he obtained a "best value" of 3.07 Langleys. In 1886, Langley became Secretary of the Smithsonian Institute in Washington and followed the tradition for the Secretary to continue his researches. He set up the Astrophysical Observatory in 1892 and remained Director of it until his death in 1906. The majority of the work done until the turn of the century concerned the development and perfection of the spectrobolometer, and the automated mapping of the enormous number of absorption lines in the apparent absorption bands. It was not until the new century that attention turned again to the spectrum as a means of obtaining the coefficient of air transmission as a function of wavelength, so as to determine the solar constant to the best of ability at that time. C. G. Abbot, then Aid to the Director, commented in the Annual Report of the Institution for 1902 [8] on the need to determine the absorption of the spectrometer itself to obtain the true distribution. "As the mirror surfaces deteriorate, this has to be done not infrequently". Clearly silvered mirrors give reduced reflection without apparent deterioration, especially at the shorter (violet and ultra-violet) wavelengths. In the Annual Report for 1903 [9], he reported that it was only since January of that year that the region of 0.375~).47 could be included in regular bolographic work. (By
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L. B. MULLETT
then the spectrobolometer had been automated to draw the spectra directly.) What is more, higher atmospheric attenuation was now being found in this region, and a computed spectrum for outside the atmosphere peaked at 0.48 #, much as we now find it. Attention turned again to determining the solar constant, and one must refer increasingly to the work of Abbot and Fowle (a "Junior Assistant"). The progress of the work is "plotted" in the annual reports of the Smithsonian Institution, where there was always a section on the Astrophysical Observatory, written by the Aid Acting in Charge (Abbot) to Langley as Director of the Laboratory and Secretary of the Institution. The first six months of both 1900 and 1901 were taken up with expeditions in connection with solar eclipses. However, by 1902, further improvements had been made in the speedy and accurate plotting of solar spectra (so avoiding problems due to changing sky conditions) and in the computation of transmission as a function of wavelength. In 1903, with these new spectra and with measurements of total radiation using a mercury pyrheliometer and a Crova alcohol actinometer, values of the solar constant were obtained from high and low sun data in Washington. The mean of results before March 1903 was 2.229 Langleys, and after that date 2.080. There was much speculation as to whether there had been a lessening of solar radiation. Nevertheless, in a paper to the Astrophysical Journal in March 1903 [10], Langley was unrepentant giving a "provisional" value of 2.54 and a firm opinion that measurements near sea-level would always give a smaller result than at elevation, quoting again his Mount Whitney results and pressing the need for an elevated solar laboratory. By June 1904 in a paper to the Astrophysical Journal [11] he gave the 1902/1903/1904 results for the solar constant as ranging from 1.93 to 2.29, but expressed doubt as to the absolute validity of the actinometry to within 20% and again pressed for an elevated station. This time the funds were forthcoming and in May 1905 Abbot and his team left for Mount Wilson with all the necessary equipment. The average value of the solar constant from measurements made at a height of 1780 m by October of that year was 2.024 Langleys. By then, Langley, though mentally active, was seriously ill and died in February 1906. He was almost certainly aware of the results, but we do not know whether he appreciated their significance. Abbot, however, thought he did. Earlier in 1903 he had reported to Langley "All the results depend upon the constants of the pyrheliometer and may therefore be subject to multiplication by a constant factor to be subsequently determined. Comparing the values
which you have given in the Mount Whitney report of 3 calories, it will be seen that they are 25% smaller, and the difference does not appear to depend on the transmission coefficient but rather seems chiefly due to a difference in actinometry. Thus you have stated that the usual actinometer reading at Allegheny, Pa., for clear blue sky at 1.7 calories, while the very highest value obtained here is 1.44 calories". But still, the fact is stranger than fiction; though it might read like it. As we have already seen, Langley first obtained a value of 3.5 Langleys from his transmission measurements on Mount Whitney and chose to call this his maximum value. He then proceeded to determine a "minimum" value, using high and low sun values of the transmission coefficients. The measurements so obtained at Lone Pine related to the whole atmosphere above Lone Pine. Langley quotes an example of a reading of 201 (unspecified units) from the bolometer at a wavelength of 0.6 p, the peak of the spectrum. Using the high and low sun transmission coefficient, the computed value for Mountain Camp was 206. However, the actual Mountain Camp reading was 249.7. Similar high and low sun observations at Mountain Camp gave 275 outside the atmosphere. Langley then applied a correction of 249.7/206, arguing that if one had been able to ascend above the atmosphere there would have been the same shortfall as at Mountain Camp from the Lone Pine calculation. The same procedure was followed at all wavelengths, giving a solar constant of 2.63 Langleys. Langley then "devised" an intermediate spectrum from which he derived his "best" value of 3.07 Langleys. There may, or may not have been a systematic error, or even a real effect in the Lone Pine/Mountain Camp data (due to the bolometer, the actinometer, the weather or the fact that the lower atmosphere up to Mountain Camp was more absorbant than the total atmosphere above Lone Pine) but the Mountain Camp high and low sun data, being relevant only to the atmosphere above the camp, should stand entirely on their own benefit. Taking the extrapolated peak of the botometer distribution outside the atmosphere as 275, it can be seen from the Langley spectrum that the solar constant would have been 2.3 Langleys. In fact, Langley tabulates solar constant values obtained in this way which average 2.22 as compared with 2.06 from high and low sun values at Lone Pine. The Mountain Camp figure of 2.22 should be taken to be the more realistic since there was less atmosphere which to be uncertain about. Had Langley then believed the absolute pyrheliometers rather than the indirect actinometers, he would have obtained a value
Langley 1834-1906 of 1.92, remarkably close to the presently accepted value of 1.94. In 1906, after Langley's death, an unidentified translator of a paper by R a d m a n on " A s t r o n o m y on M o n t Blanc" reproduced in the Smithsonian Report [12], added a footnote: Langley's value, 3 calories, depends on an inference which he made in 1881 of a failure of Bouguer's formula for allowing for atmospheric absorption, even when applied to homogenous rays. Nevertheless he made no correction in his later papers for t h i s . . . That this was most likely A b b o t can be deduced from the Annual Report for 1907 [13] where, now as Director of the Astrophysical Observatory, he wrote: The results of Langley, while seemingly in contradiction of these (results), in reality support them. For as he states at page 211 of the Report of the Mount Whitney expedition, his value (3 calories) for the "solar constant" depends upon an allowance which he made for an apparent "systematic error in high and low sun observations at one station", of such a nature as becomes manifest "by calculating at the lower station, from our high and low sun observations there, the heat which should be found at a certain height in the atmosphere, then actually ascending to this height, and finding the observed heat there conspicuously and systematically greater than the calculated one". In fact there is no such systematic error, no correction for it should have been applied by Langley, and the best mean value of his experimental determination of the "solar constant" at Mount Whitney and Lone Pine is 2.114 calories per square centimeter per minute. Finally, after a more "respectable" time interval, A b b o t wrote in the second volume of the Annals o f the Astrophysical Observatory [14]: Owing to this most unfortunate error, the great authority of Langley has supported the value of 3.0 calories for the last twenty five years, and many observers have been perhaps wrongly influenced by it. In reality, Langley's Mount Whitney observations support the value 2.1 calories. Indeed, as the last word, and with our still longer hindsight, Langley's value of the solar constant could well have been precisely 1.92 calories.
97 CONCLUSION
F o r our older scientists, there is clearly much o f research as well as personal interest to be found in the archives. F o r our younger scientists there is the moral that even though it may take another I00 years, their transgressions against scrupulous accuracy and open mindedness may still be found out. REFERENCES
1. C. Alder, Samuel Pierport Langley, Annual Report of the Board of Regents of the Smithsonian Institute for the year ending 30 June 1906. 2. S.P. Langley, The Selective Absorption of Solar Energy, Am. J. Sci. Third Series, 169-196 (1883). 3. S. P. Langley, C. R. Seances L',4cad. Sci. 95, 482-487 (1896). 4. N. Robinson, Solar Radiation. Elsevier (1966). 5. S.P. Langley, Researches on solar heat and its absorption by the earth's atmosphere. A report of the Mount Whitney Expedition (1881), Professional Peapers of the Signal Service No. XV, US War Dept., (1884). 6. W. Sehmidt, Absorption des Sonnenstrahling in Wasser, Sitzungsberichte des Mathematisch--Naturwissenschaftlichen Klasse, der Kaiserlichen Academie der Wissensehaften, CXVII Band, Abteihing IIa, Jahrgang 1908, Heft IBis X, pp. 237-253 (1908). 7. P. T. Tsilingiris, Analytical and experimental studies on Salt Gradient Solar Ponds. Ph.D. Thesis, University of Reading (1985). 8. C. G. Abbot, Report of the work of the Astrophysical Observatory, Annual Report of the Board of Regents of the Smithsonian Institution, for the year ending 30 June 1902. 9. C. G. Abbot, ibid., to 30 June 1903. 10. S. P. Langley, The solar constant and related problems, Astrophys. J. XVII, No. 2, March 1903. 11. S. P. Langley, On a possible variation of the solar radiation and its probable effect on terrestrial temperatures, Astrophys. J. XIX, No. 5, June 1904. 12. Unknown translator of a paper by H. Radau, Astronomy on Mont Blanc, Annual Report of the Board of Regents of the Smithsonian Institution, for the year ending 30 June 1906. 13. C. G. Abbot, Report of the work of the Astrophysical Observatory, Annual Report of the Board of Regents of the Smithsonian Institution for the year ending 30 June 1907. 14. C. G. Abbot, Annals of the Astrophysical Observatory of the Smithsonian Institution, Vol. 11, for the years 1900-1907.