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James Joule and meteors
Vistas in Astronomy, Vol. 33, pp. 143--148, 1990 Printed in Great Britain. All rights reserved.
0083--6655/90 $0.00 + ..50 ~ 1990 Pergamon Press pie.
JAMES JOULE A N D METEORS David W. Hughes Department
o f Physics, T h e U n i v e r s i t y , S h e f f i e l d $3 7 R H ,
U.K.
Abstract. 1989 was the hundredth anniversary of the death of James Prescott Joule, the Prescott being his mother's family naane and the Joule, rhyming with cool, originating from the Derbyshire village of Youlgreave. Joule is rightly famous for his experimental efforts to establish the law of conservation of energy, and for the fact that J, the symbol known as the mechazfical equivalent of heat, is named after him. Astronomically his "light has been hidden under a bushel". James Joule had a major influence on the physics of meteors. Joule was born on Christmas Eve 1818 into a famous Manchester/Salford brewing family. He was educated at home and once his tutelage was over he went into the farnily firm where he not only helped run the business but also started to carry out a series of scientific experiments. Eventually his scientific interests predominated but it has been said that the requirements of brewing technology and the accountancy needed to run a business helped to mould his scientific attitudes. A spinal weakness at birth turned him into a hunchback, and this shy and unassertive man was always sensitive about his public appearances. In Joule's time science was changing front being the affair of the gentleman devotee to being the occupation of the full-time professional ensconced in the university laboratory. Joule was firmly in the first category. Almost all of his research was carried out in his own laboratory, first at the brewery and then at his home, and it was carried out at his own expense. For an excellent biography of Joule see Cardwell I. In the 1840s shooting stars were still objects of considerable mystery. Let me briefly recap. Edmond ]talley 2 had guessed that meteors were produced by "collections of matter" that were falling into the Sun but happened to be destroyed by hitting the Earth on the way. Unfortunately, Halley changed his mind later on 3. lie had bcen looking at some rather spectacular experimental demonstrations, made to the Royal Society by the Reverend Whiteside, in which inflammable sulphurous vapours were hcated and then rose to the top of a collecting vessel. Halley then returned to the view first expressed by Aristotle - that inflammable vapours rose up from the surface of our planet, gathered at the top of the atmosphere, and at times were ignited due to the celestial revolution. The speedy burning of these vapours resulted in a meteor. Halley, the consummate scientist, did much, however, to initiate meteor physics by actually taking measurements of meteors instead of just looking on in awe and wonder, tie noted that the meteor seen from all over England on the night of March 1719 was at a height of 119 km and was moving at a speed of 8.0 km s -I . Unfortunately these measurements indicated that the meteor was "above the reputed limits of the atmosphere". This was worrying because the meteor had been accompanied by a sound "like a broadside followed by a rattle like small-arms fire". Halley knew that the passage of sound was greatly diminished in the even modest artificial vacua that could be produced in those days, and he could not understand how the sound could be transmitted from the meteor through the vacuum separating it from the top of the atmosphere. 143
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D. W. Hughes The extraterrestrial nature of the meteor precursors returned to favour due to the relationship that was established between meteors, fireballs, bolides (exploding or detonating fireballs) and the fall of meteorites. Ernst F. F. Chladni4 and Edward Howard s were mainly responsible. Chladni finally scotched the idea that meteorites were thunderstones or volcanic ejectae. Their random directions of motion suggested that the meteorite's parents were small masses of material that were travelling through space and came under the influence of the Earth's gravitational influence in such a way that they entered the atmosphere. "There, severe friction created excessive heat and electricity, which caused them to become incandescent and molten, to produce gases in their interiors, and to expand to enormous sizes until they were ruptured by explosions".7 Chladni noted that, on a priori grounds, it was just as reasonable to postulate the existence of small bodies in space as to deny their presence. With considerable perception he suggested that these objects might be rocks that failed to accrete into planets, or be ejecta from interplanetary collisions, fragments from impact events or debris from internal explosions. Howard s was a chemist who carried out the first detailed analysis of meteorites and found that meteoritic iron contained a high percentage of nickel, unlike the iron found on Earth. This again firmly pointed to the extraterrestrial nature of the meteorites. The method of measuring meteor heights using two well-spaced observers and then resorting to trigonometry, was none too accurate. Heinrich W. Brandes and Johann F. Benzenberg observed 22 meteors simultaneously between September 11 and November 4, 1798. The height at which these disappeared varied between 11 and 245 kin, the mean being 98 km (see NewtonS). The observation of shower radiants and the fact that an individual shower radiant only moved slowly against the stellar background as a function of time also underlined the fact that meteors were produced by extraterrestrial bodies.9 Alexander yon Humbolt drew attention to the radiant of the Leonids after observing the 1799 display. Denison Olmsted 1°, among others, recognised that the radiant could be caused by a perspective effect due to the Earth passing through a parallel stream of particles that were in orbit around the Sun. James Joule wrote his paper 11 on shooting stars in late 1848 and the paper seems to have been prompted by two other papers 12,13, both written by Sir John W. Lubbock, that appeared in the February and March issue of what was known as The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science. Lubbock14 was trying to explain the sudden disappearance of shooting stars. He put forward three hypotheses "for this most unusual phenomenon". 1. The body shines by its own light, and then explodes like a sky rocket, breaking into minute fragments too small to be any longer visible to the naked eye. 2. Such a body having shone by its own light suddenly ceases to be luminous. This proposal refers to the suggestion by Sir Humphry Davy is that "the luminous appearances of shooting stars and meteors cannot be owing to any inflammation of elastic fluids, but must depend upon the ignition of solid bodies. DR HALLEY calculated the height of a meteor at ninety miles, and the great American meteor which threw down showers of stones was estimated at seventeen miles high. The velocity of motion of these bodies nmst in all cases be immensely great, and the heat produced by the compression of the most rarefied air from the velocity of motion must be probably sufficient to ignite the mass; and all the phenomena may be explained, if falling stars be supposed to be small solid bodies moving round the earth in very eccentric orbits, which become ignited only when they pass with immense velocity through the upper regions of the atmosphere and if the meteoritic bodies which throw down stones with explosions be supposed to be similar bodies which contain either combustible or elastic matter".
James Joule and Meteors So according to Davy the shooting star "went out" when the meteoroid stopped burning, and the meteor's energy came from ordinary combustion. 3. The body shines by the reflected light of the sun, and ceases to be visible by its passing into the Earth's shadow. The last of these propositions is testable by actual observations and Lubbock continues his paper by developing equations that enable the distance between the end-point of the meteor and the observer to be calculated if the point on the sky at which the meteor disappeared and the time of the disappearance are kno~vn. Also if a set of observations of a singJe meteor were to hand the calculations would distinguish between the meteor being a satellite of Earth or a planetesimal in an orbit around the Sun. Note that according to this hypothesis a meteoroid in orbit around the Earth at a height of one earth radius above the surface would pass above an observer every 2 hr 56 min and could possibly be seen more than once during the night. Lubbock writes that "the question of their (meteors) origin seems to have excited more controversy than any other; but for the solution of this difficulty, I apprehend we possess no d a t a which do not apply equally to the Moon and to other bodies in the solar system". Variations in the luminosity along the path of the meteor are blamed on changes in the observer - meteor - sun geometry and on changes in the visible portion of the meteor disc. Lubbock makes no a t t e m p t to deduce how large a meteor must be to give a specific luminosity. According to Lambert A. J. Quetelet 10 the average velocity of a meteor was about 32 km s -1. This observation counted against the satellite theory which indicated that a maximum velocity of 8 km s -1 could be obtained by an orbiting body and even this required the satellite to be skimming the Earth's surface. Returning to the eclipse, Lubbock wrote "it seems to me that the splitting of the falling stars, like a rocket and the trains of light, a phenomenon often witnessed, might, if other circumstances were favourabh to the explanation, be accounted for by supposing the star to graze the surface of the shadow before absolute immersion". Lubbock 13 pondered the mechanism responsible for placing the planetesimals in orbit around the Sun. Observations of speedy variations in the sunspot activity suggested that the bodies were thrown up from the Sun's surface. The body would move off on an ellipse that would eventually bring it back to the Sun again. Jovian perturbations were invoked to prevent this. He also introduced what was to become aal important factor in meteor physics and that was the interaction between the meteoroid and the atmosphere. "The phenomena of shootin9 stars may possibly throw light upon the question of the extent to which an atmosphere extends capable of affording any sensible resistance to the motion of such bodies, and may thus afford an interesting illustration of the connection which exists between different branches of physical science". An adiabatic approach 1~ to the physical state of the atmosphere indicates that it has a finite height and does not have a density that decreases exponentially to an infinitely low value an infinite distance away. Lubbock is found that the "atmosphere ceases altogether at a height of 22.35 miles" (35.97 km). This quotation is too precise because later on Lubbock admits that he is not sure as to what value to take for the ratio between the specific heat of the atmospheric gas
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D. W. Hughes at constant pressure to that at constant volume. He takes values of 1.4,1.5 and 1.6 for this ratio and quotes atmospheric heights of 25.81, 23.896 and 22.52 km respectively. Biot TM found the height to be about 29 kin. Ivory2° disagreed with both of them and found that it extended to a much greater height. La Lande21 quoted a range of values, especially one of 70.8 km estimated by Delambre from observations of twilight. So there was some confusion as to whether meteors, with their mean height of 98 km, were actually inside or outside the atmosphere. Joule 11 firmly thought that meteors occurred in the atmosphere although he gave no indica, tion as to how he thought the atmospheric density varied as a function of height. He wrote "I have for a long time entertained an hypothesis with respect to shooting stars, similar to that entertained by Chladni (see ref. 4) to account for meteoritic stones, and have reckoned the ignition of these miniature planetary bodies by their violent collision with our atmosphere, to be a remarkable illustration of the doctrine of the equivalency of heat to mechanical power or vis viva". By using the word 'equivalence' Joule was stressing the conservation of energy. The total amount of energy is conserved independent of whether it is kinetic, potential, chemical, electrical or what have you. We take this for granted nowa~iays but in Joule's time the complete conversion of heat into work or work into heat was no more conceivable than say the conversion of gravity into hydrogen or hydrogen into gravity. Joule found that a given amount of work always produced the same amount of heat. In my early school days this was summed up by the statement that the work done in raising a weight of 1 lb (0.454 kg) through 772 ft (235 m) will, if converted completely into heat, raise the temperature of I lb (0.454 kg) of water by one degree Fahrenheit (0.56 degree Celsius). The term vis viva ill Joule's statement about meteors, can be thought of as kinetic energy - the "living force" of a moving body. It is obtained by multiplying the mass of the body by the square of its velocity. Joule's ideas are summed up perfectly by a quote from a popular lecture that he gave at St. Ann's School in Manchester on the 28th of April 1847. "You have, no doubt, frequently observed what are called shooting stars, as they appear to emerge from the dark sky of night, pursue a short and rapid course, burst, and are dissipated in shining fragments. From the velocity with which these bodies travel there can be little doubt that they are small planets which, in the course of their revolution round the sun, are attracted and drawn to the earth. Reflect for a moment on the consequences which would ensue, if a hard meteoritic stone were to strike the room in which we are assembled with a velocity sixty times as great as a cannon ball. Tlle dire effects of such a collision are effectually prevented by the atmosphere surrounding our globe, by which the velocity of the meteoric stone is checked, and its living force converted into heat, which at last becomes so intense as to melt the body and dissipate it in fragments too small probably to be noticed in their fall to the ground. Hence it is, that although multitudes of shooting stars appear every night, few meteoric stones have been found, those few corroborating the truth of our hypothesis by the marks of intense heat which they bear on their surfaces" .22
W h a t is so important about Joule's short paper is that he does much more than just present a bright idea, he actually tackles the physics of the problem.
James Joule and Meteors "The likelihood of the above hypothesis will be rendered evident, if we suppose a meteoritic stone, of the size of a six inch cube, to enter our atmosphere at the rate of eighteen miles per second of time, the atmosphere being 1/100 th of its density at the earth's surface. The resistance offered to the motion of the stone will in this case be at least 51,600 lbs. ; and if the stone traverses twenty miles with this amount of resistance, sufficient heat will thereby be developed to give 1° Fahrenheit to 6,967,980 lbs. of water. Of course by far the largest portion of this heat will be given to the displaced air, every particle of which will sustain the shock, whilst only the surface of the stone will be in violent collision with the atmosphere, tIence the stone may be considered as placed in a blast of intensely heated air, the heat being communicated from the surface to the centre by conduction. Only a small portion of the heat evolved will therefore be received by the stone; but if we estimate it at only 1/100 th, it will still be equal to 1° Fahrenheit per 69,679 lbs. of water, a quantity quite equal to the melting and dissipation of any materials of which it may be composed. The dissolution of the stone will also be accelerated in most cases by its breaking into pieces, in consequence of the unequal resistances experienced by different parts of its surface, especially alter its cohesion has been partially overcome by heat. It appears to me that the varied phenomena of meteoric stones and shooting stars may all be explained in the above manner; and that the different velocities of the meteorolltes, varying from four to forty miles per second according to the direction of their motions with respect to the earth, along with their various sizes, will suffice to show why some of these bodies are destroyed the instant they arrive in our atmosphere, and why others, with diminished velocity, arrive at the earth's surface".
James Joule got all the concepts right. Even tile general conception that objects can be cooled down by moving them through the air or allowing air to flow past them was overcome by Joule resorting to an experiment. He made air, at around five atmospheres pressure, escape through a small orifice. When he tried to squeeze and pinch the jet with his finger and thumb he felt an intense burning sensation, the heat coming from the kinetic energy of the escaping air. z3 The relationship between the magnitude of a visual meteor and the mass of the causative meteoroid was seriously tackled for the first time in 1922 by Ernst J. Opik. 24 This required an analysis of the efficiency of the conversion of kinetic energy into luminosity. The atmospheric density in the meteor region (height 80 to 110 km) varies from around 10 -5 to 10 -7 times the value at ground level. 2s The first accurate estimations of the densities of these regions came in 1922 from the analysis of meteor trains. 26 The velocity range quote by Joule was gleaned from observational data. The minimum velocity with which a meteoroid can hit the upper atmosphere is the Earth's escape velocity, 11 km s - l , and the maximum is that of a parabolic meteoroid moving in exactly the opposite direction to Earth, some 74 km s -1, (so the range is 7 to 46 miles per see). Joule's statement that the chance of survival of a meteorite during its atmospheric passage was strongly velocity dependent has also been borne out. 2r Joule well deserves the recognition of being the father of meteor physics. He was the first to place the interaction between the kinetic energy of the meteoroid and the Earth's atmosphere on a firm scientific footing. We can best conclude this short paper by quoting Joule's own words concerning the ablation of meteoroids in the atmosphere. "I cannot but be filled with admiration and gratitude for the wonderful provision thus made by the Author of nature for tile protection of his creatures. Were it not for the atmosphere which covers us with a shield, impenetrable in proportion to the violence which it is called upon to resist, we should be continually exposed to a born-
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D. W. Hughes bardment of the most fatal and irresistible character. To say nothing of the larger stones, no ordinary buildings could afford shelter from very small particles striking at the velocity of eighteen miles per second. Even dust flying at such a velocity would kill any animal exposed to it".
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17
James Joule; A Biography, by Donald S. L. CardweU, Manchester University Press, Manchester, 1989. Halley, E., Phil. 7~uns. Roy. Soc., 29, 159-164 (1714). Halley, E., Phil. Trans. Roy. Soc., 30, 978-990 (1719). Chladni, E. F. F., Ueber den Ursprung der yon Pallas gefundenen und anderer ihr ~hnlicher Eisenmassen. J. F. Hartnoch, Riga, 1794. Howard, E. C., Phil. Trans. Roy. Soc. , 92, 168-212 (1802). Cosmic Debris: Meteorites in History, by John G. Burke, University of California Press, Berkeley, 1986. p. 52 & 228. Burke, op. cit. p. 43. Newton, H. A., American Jour. Sci. , Second Series, 38, 135-141 (1864). Newton, H. A., American Jour. Sci. , Second Series, 36, 145-149 (1863). Olmsted, D., American Jour. Sci. , Second Series, 25, 363-411 and 26, 132 and 137-174. Joule, J. P., Phil. Mug. , Third Series, 32, 349-350 (1848). Lubbock, J. W., Phil. Mug. , Third Series, 32, 81-88 (1848). Lubbock, J. W., Phil. Mag., Third Series, 32,170-172 (1848). Sir John Williaxn Lubbock (1803-1865) was a banker who spent his leisure hours in scientific pursuits. He concentrated on astronomy and mathematics and was mainly interested in tidal prediction and lunar and planetary orbital perturbation. He was made a Fellow of the Royal Society in 1829 and was the first Vice-ChanceUor of the University of London (see the Dictionary of National Biography, Smith, Elder & Co. London 1898, XXXI, 227-228.) Davy, H., Phil. Trans. Roy. Soc. , (1817) see p. 75. Quetelet, L. A. J., Catalogue des Principles Apparitions d'Etoiles Filants, Brussels. In an adiabatic atmosphere the pressure, P and the density p are related by P = A f , where A is a constant and q is the ratio between the specific heat at constant pressure and that at constant volume. Integration of the hydrostatic equation (dP = -gpdh) indicates that the density falls to zero at a height, ha given by ha-
ATP~°-I
g(7- 1) where po is the density at ground level and g is the acceleration of gravity in the region. 18 Treatise on the Heat of Vapours and on Astronomical Refraction, by J . W . Lubbock, London, (1840). This memoir was then reprinted in six parts in Phil. Mag., Third Series, 16, 434441,510-514, 562-569, and 17 273-280, 467-473,488-507 (1840). 19 Biot, M., Comptes Rendus des S~ances de l'Acad~mie des Scienoes, 8, 95. 20 see ref. 18, Supplement, p. 3. 21 La Lande, J. J. F., Astronomic, Vol. 2, Article 2270, (1771). 22 see ref. 11 and also the Manchester Courier newspaper for May 12, 1847. 23 see ref. 1, p. 145. 24 Opik, E. J., Pubs. Astr. Obs. Tartu. (Dorpat), 25, No. 1 (1922). 25 Hughes, D. W. in Cosmic Dust ed J. A. M. McDonnell, John Wiley, Chichester, p. 123-185, (1978). 26 Lindemann, F. A. & Dobson, G. M. B., Proc. Roy. Soc. Lond. , A102, 411 (1922). 27 Hughes, D. W., Meteoritics, 16, 269-281 (1981).
0083--6655/90 $0.00 + ..50 ~ 1990 Pergamon Press pie.
JAMES JOULE A N D METEORS David W. Hughes Department
o f Physics, T h e U n i v e r s i t y , S h e f f i e l d $3 7 R H ,
U.K.
Abstract. 1989 was the hundredth anniversary of the death of James Prescott Joule, the Prescott being his mother's family naane and the Joule, rhyming with cool, originating from the Derbyshire village of Youlgreave. Joule is rightly famous for his experimental efforts to establish the law of conservation of energy, and for the fact that J, the symbol known as the mechazfical equivalent of heat, is named after him. Astronomically his "light has been hidden under a bushel". James Joule had a major influence on the physics of meteors. Joule was born on Christmas Eve 1818 into a famous Manchester/Salford brewing family. He was educated at home and once his tutelage was over he went into the farnily firm where he not only helped run the business but also started to carry out a series of scientific experiments. Eventually his scientific interests predominated but it has been said that the requirements of brewing technology and the accountancy needed to run a business helped to mould his scientific attitudes. A spinal weakness at birth turned him into a hunchback, and this shy and unassertive man was always sensitive about his public appearances. In Joule's time science was changing front being the affair of the gentleman devotee to being the occupation of the full-time professional ensconced in the university laboratory. Joule was firmly in the first category. Almost all of his research was carried out in his own laboratory, first at the brewery and then at his home, and it was carried out at his own expense. For an excellent biography of Joule see Cardwell I. In the 1840s shooting stars were still objects of considerable mystery. Let me briefly recap. Edmond ]talley 2 had guessed that meteors were produced by "collections of matter" that were falling into the Sun but happened to be destroyed by hitting the Earth on the way. Unfortunately, Halley changed his mind later on 3. lie had bcen looking at some rather spectacular experimental demonstrations, made to the Royal Society by the Reverend Whiteside, in which inflammable sulphurous vapours were hcated and then rose to the top of a collecting vessel. Halley then returned to the view first expressed by Aristotle - that inflammable vapours rose up from the surface of our planet, gathered at the top of the atmosphere, and at times were ignited due to the celestial revolution. The speedy burning of these vapours resulted in a meteor. Halley, the consummate scientist, did much, however, to initiate meteor physics by actually taking measurements of meteors instead of just looking on in awe and wonder, tie noted that the meteor seen from all over England on the night of March 1719 was at a height of 119 km and was moving at a speed of 8.0 km s -I . Unfortunately these measurements indicated that the meteor was "above the reputed limits of the atmosphere". This was worrying because the meteor had been accompanied by a sound "like a broadside followed by a rattle like small-arms fire". Halley knew that the passage of sound was greatly diminished in the even modest artificial vacua that could be produced in those days, and he could not understand how the sound could be transmitted from the meteor through the vacuum separating it from the top of the atmosphere. 143
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D. W. Hughes The extraterrestrial nature of the meteor precursors returned to favour due to the relationship that was established between meteors, fireballs, bolides (exploding or detonating fireballs) and the fall of meteorites. Ernst F. F. Chladni4 and Edward Howard s were mainly responsible. Chladni finally scotched the idea that meteorites were thunderstones or volcanic ejectae. Their random directions of motion suggested that the meteorite's parents were small masses of material that were travelling through space and came under the influence of the Earth's gravitational influence in such a way that they entered the atmosphere. "There, severe friction created excessive heat and electricity, which caused them to become incandescent and molten, to produce gases in their interiors, and to expand to enormous sizes until they were ruptured by explosions".7 Chladni noted that, on a priori grounds, it was just as reasonable to postulate the existence of small bodies in space as to deny their presence. With considerable perception he suggested that these objects might be rocks that failed to accrete into planets, or be ejecta from interplanetary collisions, fragments from impact events or debris from internal explosions. Howard s was a chemist who carried out the first detailed analysis of meteorites and found that meteoritic iron contained a high percentage of nickel, unlike the iron found on Earth. This again firmly pointed to the extraterrestrial nature of the meteorites. The method of measuring meteor heights using two well-spaced observers and then resorting to trigonometry, was none too accurate. Heinrich W. Brandes and Johann F. Benzenberg observed 22 meteors simultaneously between September 11 and November 4, 1798. The height at which these disappeared varied between 11 and 245 kin, the mean being 98 km (see NewtonS). The observation of shower radiants and the fact that an individual shower radiant only moved slowly against the stellar background as a function of time also underlined the fact that meteors were produced by extraterrestrial bodies.9 Alexander yon Humbolt drew attention to the radiant of the Leonids after observing the 1799 display. Denison Olmsted 1°, among others, recognised that the radiant could be caused by a perspective effect due to the Earth passing through a parallel stream of particles that were in orbit around the Sun. James Joule wrote his paper 11 on shooting stars in late 1848 and the paper seems to have been prompted by two other papers 12,13, both written by Sir John W. Lubbock, that appeared in the February and March issue of what was known as The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science. Lubbock14 was trying to explain the sudden disappearance of shooting stars. He put forward three hypotheses "for this most unusual phenomenon". 1. The body shines by its own light, and then explodes like a sky rocket, breaking into minute fragments too small to be any longer visible to the naked eye. 2. Such a body having shone by its own light suddenly ceases to be luminous. This proposal refers to the suggestion by Sir Humphry Davy is that "the luminous appearances of shooting stars and meteors cannot be owing to any inflammation of elastic fluids, but must depend upon the ignition of solid bodies. DR HALLEY calculated the height of a meteor at ninety miles, and the great American meteor which threw down showers of stones was estimated at seventeen miles high. The velocity of motion of these bodies nmst in all cases be immensely great, and the heat produced by the compression of the most rarefied air from the velocity of motion must be probably sufficient to ignite the mass; and all the phenomena may be explained, if falling stars be supposed to be small solid bodies moving round the earth in very eccentric orbits, which become ignited only when they pass with immense velocity through the upper regions of the atmosphere and if the meteoritic bodies which throw down stones with explosions be supposed to be similar bodies which contain either combustible or elastic matter".
James Joule and Meteors So according to Davy the shooting star "went out" when the meteoroid stopped burning, and the meteor's energy came from ordinary combustion. 3. The body shines by the reflected light of the sun, and ceases to be visible by its passing into the Earth's shadow. The last of these propositions is testable by actual observations and Lubbock continues his paper by developing equations that enable the distance between the end-point of the meteor and the observer to be calculated if the point on the sky at which the meteor disappeared and the time of the disappearance are kno~vn. Also if a set of observations of a singJe meteor were to hand the calculations would distinguish between the meteor being a satellite of Earth or a planetesimal in an orbit around the Sun. Note that according to this hypothesis a meteoroid in orbit around the Earth at a height of one earth radius above the surface would pass above an observer every 2 hr 56 min and could possibly be seen more than once during the night. Lubbock writes that "the question of their (meteors) origin seems to have excited more controversy than any other; but for the solution of this difficulty, I apprehend we possess no d a t a which do not apply equally to the Moon and to other bodies in the solar system". Variations in the luminosity along the path of the meteor are blamed on changes in the observer - meteor - sun geometry and on changes in the visible portion of the meteor disc. Lubbock makes no a t t e m p t to deduce how large a meteor must be to give a specific luminosity. According to Lambert A. J. Quetelet 10 the average velocity of a meteor was about 32 km s -1. This observation counted against the satellite theory which indicated that a maximum velocity of 8 km s -1 could be obtained by an orbiting body and even this required the satellite to be skimming the Earth's surface. Returning to the eclipse, Lubbock wrote "it seems to me that the splitting of the falling stars, like a rocket and the trains of light, a phenomenon often witnessed, might, if other circumstances were favourabh to the explanation, be accounted for by supposing the star to graze the surface of the shadow before absolute immersion". Lubbock 13 pondered the mechanism responsible for placing the planetesimals in orbit around the Sun. Observations of speedy variations in the sunspot activity suggested that the bodies were thrown up from the Sun's surface. The body would move off on an ellipse that would eventually bring it back to the Sun again. Jovian perturbations were invoked to prevent this. He also introduced what was to become aal important factor in meteor physics and that was the interaction between the meteoroid and the atmosphere. "The phenomena of shootin9 stars may possibly throw light upon the question of the extent to which an atmosphere extends capable of affording any sensible resistance to the motion of such bodies, and may thus afford an interesting illustration of the connection which exists between different branches of physical science". An adiabatic approach 1~ to the physical state of the atmosphere indicates that it has a finite height and does not have a density that decreases exponentially to an infinitely low value an infinite distance away. Lubbock is found that the "atmosphere ceases altogether at a height of 22.35 miles" (35.97 km). This quotation is too precise because later on Lubbock admits that he is not sure as to what value to take for the ratio between the specific heat of the atmospheric gas
145
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D. W. Hughes at constant pressure to that at constant volume. He takes values of 1.4,1.5 and 1.6 for this ratio and quotes atmospheric heights of 25.81, 23.896 and 22.52 km respectively. Biot TM found the height to be about 29 kin. Ivory2° disagreed with both of them and found that it extended to a much greater height. La Lande21 quoted a range of values, especially one of 70.8 km estimated by Delambre from observations of twilight. So there was some confusion as to whether meteors, with their mean height of 98 km, were actually inside or outside the atmosphere. Joule 11 firmly thought that meteors occurred in the atmosphere although he gave no indica, tion as to how he thought the atmospheric density varied as a function of height. He wrote "I have for a long time entertained an hypothesis with respect to shooting stars, similar to that entertained by Chladni (see ref. 4) to account for meteoritic stones, and have reckoned the ignition of these miniature planetary bodies by their violent collision with our atmosphere, to be a remarkable illustration of the doctrine of the equivalency of heat to mechanical power or vis viva". By using the word 'equivalence' Joule was stressing the conservation of energy. The total amount of energy is conserved independent of whether it is kinetic, potential, chemical, electrical or what have you. We take this for granted nowa~iays but in Joule's time the complete conversion of heat into work or work into heat was no more conceivable than say the conversion of gravity into hydrogen or hydrogen into gravity. Joule found that a given amount of work always produced the same amount of heat. In my early school days this was summed up by the statement that the work done in raising a weight of 1 lb (0.454 kg) through 772 ft (235 m) will, if converted completely into heat, raise the temperature of I lb (0.454 kg) of water by one degree Fahrenheit (0.56 degree Celsius). The term vis viva ill Joule's statement about meteors, can be thought of as kinetic energy - the "living force" of a moving body. It is obtained by multiplying the mass of the body by the square of its velocity. Joule's ideas are summed up perfectly by a quote from a popular lecture that he gave at St. Ann's School in Manchester on the 28th of April 1847. "You have, no doubt, frequently observed what are called shooting stars, as they appear to emerge from the dark sky of night, pursue a short and rapid course, burst, and are dissipated in shining fragments. From the velocity with which these bodies travel there can be little doubt that they are small planets which, in the course of their revolution round the sun, are attracted and drawn to the earth. Reflect for a moment on the consequences which would ensue, if a hard meteoritic stone were to strike the room in which we are assembled with a velocity sixty times as great as a cannon ball. Tlle dire effects of such a collision are effectually prevented by the atmosphere surrounding our globe, by which the velocity of the meteoric stone is checked, and its living force converted into heat, which at last becomes so intense as to melt the body and dissipate it in fragments too small probably to be noticed in their fall to the ground. Hence it is, that although multitudes of shooting stars appear every night, few meteoric stones have been found, those few corroborating the truth of our hypothesis by the marks of intense heat which they bear on their surfaces" .22
W h a t is so important about Joule's short paper is that he does much more than just present a bright idea, he actually tackles the physics of the problem.
James Joule and Meteors "The likelihood of the above hypothesis will be rendered evident, if we suppose a meteoritic stone, of the size of a six inch cube, to enter our atmosphere at the rate of eighteen miles per second of time, the atmosphere being 1/100 th of its density at the earth's surface. The resistance offered to the motion of the stone will in this case be at least 51,600 lbs. ; and if the stone traverses twenty miles with this amount of resistance, sufficient heat will thereby be developed to give 1° Fahrenheit to 6,967,980 lbs. of water. Of course by far the largest portion of this heat will be given to the displaced air, every particle of which will sustain the shock, whilst only the surface of the stone will be in violent collision with the atmosphere, tIence the stone may be considered as placed in a blast of intensely heated air, the heat being communicated from the surface to the centre by conduction. Only a small portion of the heat evolved will therefore be received by the stone; but if we estimate it at only 1/100 th, it will still be equal to 1° Fahrenheit per 69,679 lbs. of water, a quantity quite equal to the melting and dissipation of any materials of which it may be composed. The dissolution of the stone will also be accelerated in most cases by its breaking into pieces, in consequence of the unequal resistances experienced by different parts of its surface, especially alter its cohesion has been partially overcome by heat. It appears to me that the varied phenomena of meteoric stones and shooting stars may all be explained in the above manner; and that the different velocities of the meteorolltes, varying from four to forty miles per second according to the direction of their motions with respect to the earth, along with their various sizes, will suffice to show why some of these bodies are destroyed the instant they arrive in our atmosphere, and why others, with diminished velocity, arrive at the earth's surface".
James Joule got all the concepts right. Even tile general conception that objects can be cooled down by moving them through the air or allowing air to flow past them was overcome by Joule resorting to an experiment. He made air, at around five atmospheres pressure, escape through a small orifice. When he tried to squeeze and pinch the jet with his finger and thumb he felt an intense burning sensation, the heat coming from the kinetic energy of the escaping air. z3 The relationship between the magnitude of a visual meteor and the mass of the causative meteoroid was seriously tackled for the first time in 1922 by Ernst J. Opik. 24 This required an analysis of the efficiency of the conversion of kinetic energy into luminosity. The atmospheric density in the meteor region (height 80 to 110 km) varies from around 10 -5 to 10 -7 times the value at ground level. 2s The first accurate estimations of the densities of these regions came in 1922 from the analysis of meteor trains. 26 The velocity range quote by Joule was gleaned from observational data. The minimum velocity with which a meteoroid can hit the upper atmosphere is the Earth's escape velocity, 11 km s - l , and the maximum is that of a parabolic meteoroid moving in exactly the opposite direction to Earth, some 74 km s -1, (so the range is 7 to 46 miles per see). Joule's statement that the chance of survival of a meteorite during its atmospheric passage was strongly velocity dependent has also been borne out. 2r Joule well deserves the recognition of being the father of meteor physics. He was the first to place the interaction between the kinetic energy of the meteoroid and the Earth's atmosphere on a firm scientific footing. We can best conclude this short paper by quoting Joule's own words concerning the ablation of meteoroids in the atmosphere. "I cannot but be filled with admiration and gratitude for the wonderful provision thus made by the Author of nature for tile protection of his creatures. Were it not for the atmosphere which covers us with a shield, impenetrable in proportion to the violence which it is called upon to resist, we should be continually exposed to a born-
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148
D. W. Hughes bardment of the most fatal and irresistible character. To say nothing of the larger stones, no ordinary buildings could afford shelter from very small particles striking at the velocity of eighteen miles per second. Even dust flying at such a velocity would kill any animal exposed to it".
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
15 16 17
James Joule; A Biography, by Donald S. L. CardweU, Manchester University Press, Manchester, 1989. Halley, E., Phil. 7~uns. Roy. Soc., 29, 159-164 (1714). Halley, E., Phil. Trans. Roy. Soc., 30, 978-990 (1719). Chladni, E. F. F., Ueber den Ursprung der yon Pallas gefundenen und anderer ihr ~hnlicher Eisenmassen. J. F. Hartnoch, Riga, 1794. Howard, E. C., Phil. Trans. Roy. Soc. , 92, 168-212 (1802). Cosmic Debris: Meteorites in History, by John G. Burke, University of California Press, Berkeley, 1986. p. 52 & 228. Burke, op. cit. p. 43. Newton, H. A., American Jour. Sci. , Second Series, 38, 135-141 (1864). Newton, H. A., American Jour. Sci. , Second Series, 36, 145-149 (1863). Olmsted, D., American Jour. Sci. , Second Series, 25, 363-411 and 26, 132 and 137-174. Joule, J. P., Phil. Mug. , Third Series, 32, 349-350 (1848). Lubbock, J. W., Phil. Mug. , Third Series, 32, 81-88 (1848). Lubbock, J. W., Phil. Mag., Third Series, 32,170-172 (1848). Sir John Williaxn Lubbock (1803-1865) was a banker who spent his leisure hours in scientific pursuits. He concentrated on astronomy and mathematics and was mainly interested in tidal prediction and lunar and planetary orbital perturbation. He was made a Fellow of the Royal Society in 1829 and was the first Vice-ChanceUor of the University of London (see the Dictionary of National Biography, Smith, Elder & Co. London 1898, XXXI, 227-228.) Davy, H., Phil. Trans. Roy. Soc. , (1817) see p. 75. Quetelet, L. A. J., Catalogue des Principles Apparitions d'Etoiles Filants, Brussels. In an adiabatic atmosphere the pressure, P and the density p are related by P = A f , where A is a constant and q is the ratio between the specific heat at constant pressure and that at constant volume. Integration of the hydrostatic equation (dP = -gpdh) indicates that the density falls to zero at a height, ha given by ha-
ATP~°-I
g(7- 1) where po is the density at ground level and g is the acceleration of gravity in the region. 18 Treatise on the Heat of Vapours and on Astronomical Refraction, by J . W . Lubbock, London, (1840). This memoir was then reprinted in six parts in Phil. Mag., Third Series, 16, 434441,510-514, 562-569, and 17 273-280, 467-473,488-507 (1840). 19 Biot, M., Comptes Rendus des S~ances de l'Acad~mie des Scienoes, 8, 95. 20 see ref. 18, Supplement, p. 3. 21 La Lande, J. J. F., Astronomic, Vol. 2, Article 2270, (1771). 22 see ref. 11 and also the Manchester Courier newspaper for May 12, 1847. 23 see ref. 1, p. 145. 24 Opik, E. J., Pubs. Astr. Obs. Tartu. (Dorpat), 25, No. 1 (1922). 25 Hughes, D. W. in Cosmic Dust ed J. A. M. McDonnell, John Wiley, Chichester, p. 123-185, (1978). 26 Lindemann, F. A. & Dobson, G. M. B., Proc. Roy. Soc. Lond. , A102, 411 (1922). 27 Hughes, D. W., Meteoritics, 16, 269-281 (1981).