of an Arc of the Meridian. 415 
the stations, so far from the zenith as 6°, it is obvious little in- 
accuracy can have resulted from the difference of temperature 
here spoken of ; and this supposition will receive farther sup- 
port, from the actually near approach of the two temperatures 
to an equality with each other, as appears by taking the mean 
results of the last two columns in the register of observations. 
That the scrupulous mind may be satisfied in this particular, 
I shall insert, in its proper place, a table for supplying the 
correction arising from this cause; as the effect of a greater 
heat in the upper part of the tent is an error in excess, so a 
reverse of the case produces one in defect. 
On the first convenient opportunity, I measured, with great 
care, the distances between every successive set of dots on the 
divided arch, contained between zero and 7 0 10'. This was done 
at a time when the thermometers denoted a perfect uniformity 
in the temperature of the air within the tent, and when, from 
the calmness of the day, no streams of air could affect the 
plumb-line. Although I had, previously to the performance of 
this matter, perfectly satisfied myself that the rays of heat, 
emitted from the lamp illuminating the face of the arch, do not 
expand it perceivably, yet I thought it best to wait for a day 
when the strength of the light should enable me to discover, 
and properly bisect, the points, without the aid of that lamp. 
Between zero and 7 0 10', on the left hand arch, I found there 
were 430 revolutions of the micrometer-screw 4-38,2 divisions; 
and, between the same point and 7 0 io' on the right hand, 430 
revolutions 4- 3 9,2 divisions. 
From this it appears, that the mean value of one revolution of 
the screw, is o' 59", 038. Mr. Berge endeavoured to place the arch, 
carrying the apparatus of the micrometer, so that one revolution 
3H 2 
