548 Mr. W. J. M. Raukiue on the Law of the ''^^^''^' 



If the experiment be made by adopting the length ^ for aft, 



that of water being 1, according to the theory of emission the 

 times occupied by the two bundles of light in passing over these 

 spaces will be equal, and consequently the deviations will be 

 equal. By the other theory, on the contrary, the times occupied 

 by the light in passing throiigh both media Avill be very different ; 

 these times will be for water and for air in the ratio of 16 to 9, 

 and the deviations will be in the same ratio. 



To coincide with the one or the other theory, it will therefore 

 be sufficient to prove, either that the deviations are equal, or 

 that one is nearly double the other. 



If the equivalent lengths calculated from the theory of undu- 

 lation l:)e taken, the results will be similar, but inverse. 



According to the theory of emission, the deviations will be in 

 the ratio of 16:9; according to the other theory, they will be 

 equal. 



We have made these two experiments, and the results obtained 

 are very exact. The phsenomena observed are altogether in ac- 

 cordance -ttath the theory of undulation, and in manifest oppo- 

 sition to the theory of emission. 



In the first arrangement the deviation is greater for water than 

 for air ; it is nearly double. The difference is sensible with a 

 velocity of 400 or 500 revolutions per second ; with a velocity of 

 1500 revolutions it becomes quite evident. 



In the second arrangement the deviation is the same for air 

 and water ; and whatever be the velocity of the mirror, there is 

 no sensible difference between the two deviations. 



These experiments have been made in the meridian room of 

 the observatoiy ; the column of water was 2 metres long, and 

 was contained in a crystal tube closed at the ends with glass. 

 This length is more convenient than that which we at first em- 

 ployed, namely 3 metres. The light is less weakened, and, after 

 its double passage, retains an intensity which may be estimated 

 at double of that which was obtained with the tube of 3 metres. 



The deviations were observed at a distance of 1"50 millim. from 

 the rotating mirror. 



LXXVIII. On the Law of the Compressibility of Water at different 

 Temperatures. By^N . J. Macquorn Rankine, F.R.S.E. ^t.* 



HAVING lately had occasion, in the course of some theoreti- 

 cal researches on the velocity of soimd in liquids, to en- 

 deavour' to represent the experiments of M. Grassi on the com- 



* Communicated by the Author, having been read to the Royal Society 

 of Edinburgh, April 7\ 1851. 



