72 



Lord Eayleigh. 



connection passage K, When the mercury reaches L, the time 

 measurement is closed. 



One of the points to be kept in view in designing the apparatus is 

 to secure long enough time of transpiration without unduly lowering 

 the driving pressure. At the beginning of the measured transpiration 

 the pressure in A was about 30 cm. of mercury above atmosphere, and 



Fm. 1. 



that in B about 2 cm. below atmosphere. At the end the pressure in 

 A was 20 cm., and in B 3 cm., both above atmosphere. Accordingly 

 the driving pressure fell from 32 to 17 cm. 



Three, or, in the case of hydrogen, five, observations of the time 

 were usually taken, and the agreement was such as to indicate that the 

 mean would be correct to perhaps one-tenth of a second. The time for 

 air at the temperature of the room was about ninety seconds, and for 

 liydrogen forty-foui seconds, but these numbers are not strictly com- 

 parable. 



