TRANSACTIONS OF SECTION A. 581 



tlie plate, and the stoj) be then removed for a brief interval, it is shown by experi- 

 mental measurement that the bright star is accurately represented on the plate. 



5. On the Turbulent Motion of Water between Two Planes. 

 By Professor Sir W. Thomson, LL.I)., F.B.S. 



'6. On the Theory of Electrical Endosmose and other Allied Phenomena, and 

 on the Existence of a Sliding Coefficient for a Fluid in contact with a 

 Solid. By Professor Horace Lamb, M.A., F.B.S. — See Reports, p. 495. 



7. On the Vortex Theory of the Luminiferous JEther. 

 By Professor Sir W. Thomson, LL.D., F.B.S.— See Reports, p. 486. 



8. On the Batio of the Tivo Elasticities of Air. 

 By Professor Silvanos P. Thompson, D.Sc. 



The method suggested for determining the ratio of the two elasticities was a 

 moditication of that of ()l6ment and Desormes. A known additional volume of 

 air was suddenly introduced into a large glass flask by means of a piston in a 

 cylinder, the rise of pressure which resulted being observed in a manometric gauge — 

 first, before the heat had had time to escape ; and, secondly, after the initial tem- 

 perature had been recovered. Tlie author pointed out the utility of this form of 

 apparatus for the teaching of elementary thermodynamics, and showed that this 

 ratio was nothing else than the ratio between the slope of the adiabatic and that of 

 the isothermal drawn through any point of the pressure-volume diagram. 



The following is the proof of the proposition. 



We have as the adiabatic and the isothermal laws respectively — 



pv=p^v, = b ...... (1), 



p'V=pjV/ = a (2), 



whence 



"-■PA" _,.-! .... 



b p/o^ 



Differentiating (1) and (2) with respect to y, and dividing one result by the 

 other, we get — 



dp' a 4. , 

 dp b^ ' 



^whence finally 



dp' _ 

 dp~^' 



(3). 



9. A Null 21ethod in Electro-calorimetry . 

 By Professor W. Stroitd, D.Sc, B.A., and W. W. Haldane Gee, B.Sc. 



The method consists in dividing a current between two calorimeters in such a 

 way that the same temperature is maintained in each, as tested either by a thermo- 

 electric or bolometric arrangement. When the calorimetric capacity of the two 

 calorimeters and their contents are adjusted to equality, the corrections for cooling 

 and for the capacity of the calorimeters vanish (see ' Electrical Review,' vol xxi. 

 p. 262; 'Nature,' vol. xxxvi. p. 523). The method is found to be susceptible of 

 great acciu'acy. 



