306 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
pressure £ {P+p), at which transpiration takes place. In this way the 
occurrence of diffusion across the walls of the tube is obviated and the 
undistorted phenomenon is observable. Indeed, the envelope of porce- 
lain tube to surround the platinum capillary apparatus is advisable in 
all cases where the pyrometer is directly exposed to flame or under 
other circumstances in which the permeation discrepancy is to be ; 
apprehended. 
(960) 
