204 Prof. Silas W. Holman on the Effect of 



change of the pressure within the apparatus. For the suc- 

 cessful working of the apparatus, a proper proportioning of 

 parts to the work to be done is of course necessary, and this is 

 mainly to be accomplished by varying the size of the point to 

 which the fine tube is drawn out. The sensitiveness of the 

 apparatus may be increased by using a U-tube, instead of a 

 vertical straight tube with cistern, and allowing the fine tube 

 to extend into the open arm of this tube. A stricture at some 

 point of the U-tube reduces the too sudden fluctuations. 

 Various other modifications will readily suggest themselves to 

 persons using the apparatus. The regulator is of course of 

 service in other ways than merely rendering uniform the 

 action of the aspirator. It is possible to maintain with it a 

 constant exhaustion of any desired amount up to the full ex- 

 haustion which the aspirator can produce when working under 

 the smallest head of water likely to occur ; and in the form of 

 a U-tube, with various lengths of tube D E, to be inserted at 

 A and raised or lowered, it affords a very convenient means 

 of varying the exhaustion at will. 



Critique of the Method. 



The quantity deduced as the result of each experiment of 

 the present investigation is a ratio between the coefficients of 

 viscosity of a gas at two different measured temperatures. As 

 I have already shown*, this ratio is expressible by the follow- 

 ing equation, 



Vt _R 2 % y<?-p£ 1 + 3A* 



where 



V ' 7] %% Pi 2 -p 2 2 l+OLt 



r) t = coefficient of viscosity of the gas at f C. ; 



Vo — » » j) » 0° 0. ; 



R 2 = radius, and X 2 = length, of second capillary at t° ; 



Rj= radius, and X x = length, of first capillary at 0° ; 



p x — pressure of gas at entrance to first capillar}^ ; 



p 2 = „ ,, exit from first capillary, which is 



the same as that at entrance to second ; 

 p 3 = pressure at exit from second capillary ; 

 t = temperature of second capillary ; 

 0°= „ „ first ,, 



A = coefficient of linear expansion of the glass ; 

 a = mean coefficient of expansion of the gas between 0° 



and t°, and under the pressure p 2 . 



* Phil. Mag, iii. p. 81 (1877). 



