﻿334 ON EXTENDING A GEOGRAPHICAL SURVEr 



54'. 6, V. In these distances, I did not compute on 

 the chords of the arcs, because the instrument I had 

 in use was not sufficient for that purpose. 



E^vperiments for determining the Expansion of the 



Chain. 



In making allowance for the expansion of the 

 chain, in tlie annexed table, it will appear that I 

 have differed both from General Roy and Colo- 

 nel Williams. It may therefore be necessary to 

 give the following account of the experiments which 

 were made for ascertaining that allowance, — which 

 experiments were made by the chain itself, observ- 

 ing its length at sun-rise and at one o'clock, be- 

 tween which hours the base was generally measured. 



After the chain was extended in the coffers, in 

 the manner formerly mentioned, it was carefully ad- 

 justed, at each end, to some particular marks on the 

 register heads, about the hours of sun-rise. The 

 finsrer screw of one of these brass sliders had been 

 previously graduated into eight equal parts, on its 

 circumference, which were, counted, on its being 

 turned, by another mark on the end of the slider, 

 touching that part of the circumference. This 

 fino-er screw was observed to make 26 revolutions in 

 one inch, so that one of the divisions, on the cir- 

 cumference, was equal -rir part of an inch. Things 

 being thus adjusted, the experiments were made in 

 the following order, and the mean temperatuie taken 

 from three of the best thermometers I had, which 

 remained the whole time in the coffers, with the 

 chain ; and these coffers were covered, in the same 

 manner as they had been during the operations of 

 the measurement. 



December 11th, at one P. M. the temperature 

 was 95^ 



December 12th, at seven A.M. the mean tem- 

 perature 



