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A new photometric unit
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Edit,,ri(d.
cover k('eps the substances in place. The moistened cotton is designed effectually to arrest, hy its adhesive action, the carbon particles constituting the smoke ; and the clJarcoal, by its surface action, to condense the more dangerous and irri~ating vapors, mainly hydro-carbons, produced I)5, imperfect co~lbustion. The lime is for the absorption of carbonic acid ; but, in an atmosph(.re filled with smoke, this substance is never present in (langerou~ quantity, and this last layer may therefore, ~.xcept in special cases ~in wells, mines, &c.), be safely dispensed with. This instrument, I~urnistn.d with a suitablc hood, has been thor,)ughly tested by thc Engin(.er of the London Fire Department, and found completely to answer the purpose of its designer. A New Photometric 13"nit.~--Prof. John C. Draper proposes a new unit for photometric purposes, which, besides being theoretically much more scientific than that at present commonly used~ offers no difficulties for its adoption in practice. The method of Bunsen is that usually employed, and depends upon the determination of the relative intensities of two lights, one of which is supposed to be invariable. The invariable unit in this method is the light produced by the burning of" a candle ; which is defin(~d as a sperm candle of six to the pound burning at the rate of 120 grains per minute. It will be evident, upon a little reficction, that this plan is liable to serious errors in practice, fr,)m the fact that candles can never be obtained possessing the same composition, from which it follows that the light emitted in a given time by two candles with the same consumption by weight, will be ~ variable, and not as the theory demands, a constant factor. The plan suggested by lh'oL Drawer avoids this error completely ; and consists in the adoption of the light emitted by an incandescent solid--a platinum coil--heated to a certain temperaturc. With a given substance, a light of a definite and measurable intensity is emitted at different temperatures. The invariability of the light from this source, at any convenient degree of' heat, affords a satisfactory unit ; it is only necessary that the flame cmployed to heat the coil shallbe itsclf non-luminous. The course adopted by the author is to allow a flame of pure dry hydrogen, burning at a definite rate, to impinge upon a platinum coil ; and it was found on experiment, that so long as wire of the same * Sci. Amer., October, 1871.
Items and Novelties.
369
diameter was used in constructing coils of the same dimensions, and those were subjected t~ a flame of hydrogen burning at the same rate, the light emitted by the glowing wire was of the same intensity. There can be ~o doubt that measurements made upon this method will be far more reliable than those made in the usual way.
T h e S p e c t r u m o f t h e A u r o r a . - - Prof. Geo. F. Barker. Through the kindness of the author, we arc in receipt of advanced sheets of the Am. Journal of ,b'cic~ce, containing the results of his observation of tim beautiful crimson aurora of November 9th, a fine display of which was visible at this place. The observation resulted in one point of great interest, namely, the detection of a line in the auroral spectrum not hitherto observed. The following tabulation of the observation, with the accompanying explanation, will suffice to present ti~e essential portion of the paper : Line B C (1) D (2) E (3) h (4) F (5) G
Scale Number 76 82 90 100 110.5 124"5 130 130 138 1{6"5 145 189
Wave Length 687 656 623 589 562 59.7 517 517 502 48~; 482 431
Auroral Lines
Other measurements.
(;23
627 Zbllner
5(;2
557 AngstriSm
517
520 Winlock
502 482
485 Alvan Clark, J r .
In this table, column 1 gives the auroral and the Fraunhofer lines ; column 2, the number of these as mc:~sured upon the scale of the spectroscope used ; column 3, the wave lengths of these lines ; column 4, the wave lengths of the auroral lines ; and column 5, the wave lengths of auroral lines noticed and measured by other observers, and which the autimr assumes to be identical with those opposite to them in his own obserwttion. I t will be seen that the line (4), of wave length 502 is a new one, since it appears not to have been noticed by former observers.* In a note appended to the above, Professor Barker mentions that the last number of the Astronomische Nachrichten (No. 1864), received since his notice was in type, contains an account by Vogel of the spectra of auroras seen at
Edit,,ri(d.
cover k('eps the substances in place. The moistened cotton is designed effectually to arrest, hy its adhesive action, the carbon particles constituting the smoke ; and the clJarcoal, by its surface action, to condense the more dangerous and irri~ating vapors, mainly hydro-carbons, produced I)5, imperfect co~lbustion. The lime is for the absorption of carbonic acid ; but, in an atmosph(.re filled with smoke, this substance is never present in (langerou~ quantity, and this last layer may therefore, ~.xcept in special cases ~in wells, mines, &c.), be safely dispensed with. This instrument, I~urnistn.d with a suitablc hood, has been thor,)ughly tested by thc Engin(.er of the London Fire Department, and found completely to answer the purpose of its designer. A New Photometric 13"nit.~--Prof. John C. Draper proposes a new unit for photometric purposes, which, besides being theoretically much more scientific than that at present commonly used~ offers no difficulties for its adoption in practice. The method of Bunsen is that usually employed, and depends upon the determination of the relative intensities of two lights, one of which is supposed to be invariable. The invariable unit in this method is the light produced by the burning of" a candle ; which is defin(~d as a sperm candle of six to the pound burning at the rate of 120 grains per minute. It will be evident, upon a little reficction, that this plan is liable to serious errors in practice, fr,)m the fact that candles can never be obtained possessing the same composition, from which it follows that the light emitted in a given time by two candles with the same consumption by weight, will be ~ variable, and not as the theory demands, a constant factor. The plan suggested by lh'oL Drawer avoids this error completely ; and consists in the adoption of the light emitted by an incandescent solid--a platinum coil--heated to a certain temperaturc. With a given substance, a light of a definite and measurable intensity is emitted at different temperatures. The invariability of the light from this source, at any convenient degree of' heat, affords a satisfactory unit ; it is only necessary that the flame cmployed to heat the coil shallbe itsclf non-luminous. The course adopted by the author is to allow a flame of pure dry hydrogen, burning at a definite rate, to impinge upon a platinum coil ; and it was found on experiment, that so long as wire of the same * Sci. Amer., October, 1871.
Items and Novelties.
369
diameter was used in constructing coils of the same dimensions, and those were subjected t~ a flame of hydrogen burning at the same rate, the light emitted by the glowing wire was of the same intensity. There can be ~o doubt that measurements made upon this method will be far more reliable than those made in the usual way.
T h e S p e c t r u m o f t h e A u r o r a . - - Prof. Geo. F. Barker. Through the kindness of the author, we arc in receipt of advanced sheets of the Am. Journal of ,b'cic~ce, containing the results of his observation of tim beautiful crimson aurora of November 9th, a fine display of which was visible at this place. The observation resulted in one point of great interest, namely, the detection of a line in the auroral spectrum not hitherto observed. The following tabulation of the observation, with the accompanying explanation, will suffice to present ti~e essential portion of the paper : Line B C (1) D (2) E (3) h (4) F (5) G
Scale Number 76 82 90 100 110.5 124"5 130 130 138 1{6"5 145 189
Wave Length 687 656 623 589 562 59.7 517 517 502 48~; 482 431
Auroral Lines
Other measurements.
(;23
627 Zbllner
5(;2
557 AngstriSm
517
520 Winlock
502 482
485 Alvan Clark, J r .
In this table, column 1 gives the auroral and the Fraunhofer lines ; column 2, the number of these as mc:~sured upon the scale of the spectroscope used ; column 3, the wave lengths of these lines ; column 4, the wave lengths of the auroral lines ; and column 5, the wave lengths of auroral lines noticed and measured by other observers, and which the autimr assumes to be identical with those opposite to them in his own obserwttion. I t will be seen that the line (4), of wave length 502 is a new one, since it appears not to have been noticed by former observers.* In a note appended to the above, Professor Barker mentions that the last number of the Astronomische Nachrichten (No. 1864), received since his notice was in type, contains an account by Vogel of the spectra of auroras seen at