334 PKOCEEDINGS OP THE AMERICAN ACADEMY 



ature of C2°, the times required for them to reach theu' normal length 

 will be about as follows : — 



T requires from 15 to 20 minutes. 



C S. requires from 30 to 40 minutes. 



i?j requires about 2 hours. 



i?2 and G require about 4 hours. 



Conversely, the times within which an increase of temperature amount- 

 iug to one degree, a& indicated by the thermometer, will be inappreciable 

 in the com^Darisons, are about as follows : — 



The Tresca bar, T, seems to possess decided advantages over every 

 other form for the usual conditions under which observations are 

 made. However sudden the change of temperature, and however 

 great the amount of the change, the thermometer will indicate the true 

 length of the bar, if an observed temperature can remain constant for 

 about eleven minutes after the mercury reaches the stationary point. 

 On the other hand, standards having a large cross-section are to be 

 preferred, if a constant temperature can be maintained for several 

 hours, since the effect of any change can be neglected for a consider- 

 able time. 



Instead, therefore, of arranging protecting screens for the compar- 

 ator, I have endeavored to arrange the adjustments in such a manner 

 that a complete set of comparisons can be made before the heat devel- 

 oped by the presence of the observer could produce any effect. In 

 the comparisons described in this paper, the bar T, having the least 

 mass, is placed in front, and the microscopes are left in adjustment 

 upon the defining lines from the jji'ovious observation. The time re- 

 quired to complete the observations upon this bar is not, therefore, 

 over one or two minutes. Bar C. S., being the next in order of mass, 

 is observed next in order of time, and bar R.^, having the greatest mass, 

 is observed last. Bat after the observations are completed, the compar- 

 ing-room rernains closed for at least four hours. During this time, the 

 inci-ease of temperature due to the presence of the observer, which is 



