238 MEASUREMENT OF HIGH TEMPERATURES. Ibull.54. 
thermal data in the region of high temperatures by further data of ar 
absolute kind, which at the present state of our knowledge must alsc 
be regarded as approximate. The first steps of a method by which 
rigorously accurate data may be reached the present chapter fullv 
elucidates. 
For purposes of ordinary high- temperature measurement the con- 
stant-pressure method of air thermometry must undoubtedly be pre- 
ferred. It is superfluous to reiterate the many reasons which the text 
contains. But for the ulterior and purely scientific purposes of study- 
ing laws relative to the expansion of gases at high temperatures, both 
methods are equally valuable, and it is highly probable that an investi- 
gation of the thermal-expansion phenomena of one and the same gas 
in all admissible states of tenuity will throw more light on the subject 
in hand than an inquiry into the analogous behavior of different gases. 
Fortunately, in ail such comparisons the stem errors so nearly counter- 
balance each other as to make it probable that the measurements can 
be made with great nicety. 
(892) 
