﻿On Attraction caused by Vibrations of the Air, 279 



consideration. But if the object be to make a complete com- 

 parison between two lights, it is often more convenient to leave 

 the widths of the slits arbitrary, and then, by sliding the card, to 

 seek that part of the spectrum which allows a match. It was in 

 this way that the observations on the light of the sky were made. 

 To give an idea of the degree of accuracy to which the compa- 

 risons may be made, I may mention that in my experiments on 

 absorption, the means of six observations were usually correct 

 to about one in 50 or 60. In the less-luminous parts of the 

 spectrum the error might be somewhat greater. 



The difficulty, however, of getting a satisfactory result with 

 the blue of the sky does not lie in the inaccuracy of the measure- 

 ments, but in the arbitrary character of the light with which it 

 is compared. In order to test the theory in a strict manner, the 

 second light ought to be similar in composition to that which 

 lights up the sky. Now the sky is lit not only by the direct 

 rays of the sun, but also by itself and by the bright surface of 

 the earth. It is evident, therefore, that the requirements of the 

 case are very imperfectly met by taking as the second light that 

 of the sun as received by us, even if the translucent material 

 through which we diffuse it effects no change in the quality. A 

 nearer approximation to what we want would probably be found 

 in the diffused light of a thoroughly cloudy day. But here we 

 meet with an experimental difficulty; for the method described 

 is only available to compare two lights both given at once. A 

 suitable artificial light might no doubt be used as a middle term 

 to be afterwards eliminated ; but a candle or a lamp would hardly 

 be available, on account of the yellowness of their light. On the 

 other hand, the bluer radiation from burning magnesium would 

 probably be inconvenient, and difficult to keep constant in quality 

 from day to day. I am, however, in hopes that, by a method 

 founded on a different principle, I may be able to compare the 

 blue of one day's clear sky with the white light from the clouds 

 on another. 



Terling Place, Withamj 

 March 6, 1871. 



XXXVII. On Attraction caused by Vibrations of the Air. 

 By Professor Challis, M.A., F.R.S., F.R.A.S* 



APTER concluding from reasoning founded on hydrodyna- 

 mical principles, in my work ' On the Principles of Ma- 

 thematics and Physics/ that the vibrations of an elastic fluid, 

 such as the air, are capable of causing a permanent motion of 



* Communicated by the Author. 

 U2 



