98 



PROCEEDINGS OF THE AMERICAN ACADEMY 



TABLE VI. — Comparison by Welsh, 1852. 



It is seen that the Kew standards and the Fastre agree perfectly, but 

 that the Troughton and Simms standard stands above the Kew ther- 

 mometers at 100° F. 



The Geissler standard was made by Geissler of Bonn, and its scale 

 was on a piece of milk glass, enclosed in a tube with the stem. The 

 calibration was fair, the greatest error being about 0°.01o C, at 

 50° C. ; but no correction for calibration was made, as the instrument 

 was only used as a check for the other thermometers. 



3. Results of Comparison. 

 Calculation of Air Thermometer. 



This has already been described, and it only remains to discuss the 

 formula and constants, and the accuracy with which the different 

 quantities must be known. 



The well-known formula for the air thermometer is 



\ + bf 



r= i g - i+ fi 



»££<■ 



v\ 1 



l + bt' 



+ at' 



1-f- bt 



1 + 



-A 



at ) 



Solving with reference to T, and placing in a more convenient form, 



we have 



1 



n — h> + 



T : 



V H l + yt 



nearly, 



h '-v H iTTt 



— ii- 



