126 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



of adjustments were made. Thus, since the lower cross was always set 

 at the height of the permanent cross-hair in the lower telescope, the 

 whole expansion of the tube was observed by the movement of the mi- 

 crometer which bore the cross-hair iu the upper telescope. The tem- 

 perature of the stream of water was noted at the time of reading, and 

 also the temperature of the cathetometer was taken iu order to correct 

 for the thermal expansion of this instrument, iu case there had been a 

 slight change. 



The total distances between the two crosses was determined by direct 

 reference through the telescopes to the brass meter stick to be 611.3 mm. 

 In the telescopes a magnification was used greater than the standard 

 adopted for all other determinations. It was necessary therefore to 

 refer the micrometer dial to the brass meter stick. One dial division 

 was found to equal 0.0886 mm. 



Two series of determinations of the expansion of the tube were made 

 — the first in the manner described above; but in the second the cotton- 

 wool insulation was replaced by a glass jacket which was filled succes- 

 sively with melting ice and with warm water. In this determination the 

 temperature of the tube was taken as the average of that of the stream 

 within and of the water of the jacket without. The two were of course 

 very near together. The latter measurements are rather the more satis- 

 factory, because the temperature was more certain. 



Expansion of Glass. 



