6 



Lord Bayleigh, 



[Jan. 17, 



(/A = 1) of tlie gases. The process may be repeated backwards and 

 forwards any number of times, so as to eliminate in great degree 

 err ors of the settings and of the pressure readings. 



During these observations a curious effect was noticed, made 

 possible by the independent action of the parts of the object-glass 

 situated at various levels, as already referred to. When the bands 

 were stationary, they appeared straight, or nearly so, but when in 

 motion, owing to changes of pressure, they became curved, even in 

 passing the fiducial position, and always in such a manner that the 



I^. 



To pump. 



Scale = 4 



Fig. 2. 



ends led. The explanation is readily seen to depend upon the 

 temporary changes of temperature which accompany compression or 

 rarefaction. The full effect of a compression, for example, would not 

 be attained until the gas had cooled back to its normal temperature, 

 and this recovery of temperature would occur more quickly at the 

 top and bottom, where the gas is in proximity to the metal, than in the 

 central part of the tube. 



The success of the measures evidently requires that there should 

 be no apparent movement of the bands apart from real retardations 



