of an Arc of the Meridian. 
than A, the length of which, according to Mr. 
Ramsden's determination, may be taken = 100 feet 
-}- 0,1236 inches, in the temperature of 54 0 ; which 
gives A 0,12363 parts of an inch too long. There- 
fore, if to this is added half the wear, viz. 0,00192 
parts of an inch, we shall get -- 1 y 2 5 , 5 -, which x 2 39 
gives 2,709 feet, which add - 
The 50-feet chain, before the measurement, was 
compared also with B, and found to be 24 divisions 
on the micrometer-head longer; therefore, — I2 43 x4 
= 0,0314 parts of a foot, which likewise add 
Again, the sum of all the degrees shown on the 
4 2 5 
Feet. 
+ 2,709 
+ o>°3 1 
thermometers was 98083, wherefore, 
54 x 
263,38 x 
0,0075 
3,3713 feet, is the correction for 
the mean heat in which the base was measured above 
54 0 , the temperature in which the chains werp laid 
off, and this also add - - - - -f- 3,371 
Finally, for the reduction to the temperature of 
62°, or 8° on the brass scale, we have — - I237 X ^ 3 ’ 38 - — 
= 1,720 feet, which subtract - - — 1,720 
Hence we have the true length of the base, in the 
temperature of 62°, = r 26342,712 
The surface of the ground on which this . base was horizon- 
tally measured, is said to be not more than 35 feet above the 
surface of the sea, in the mouth of the Humber, at half tide. 
And, although it may not perhaps be a very correct deduction, 
yet, as I understand that conclusion arose out of a levelling 
operation, it may be taken for granted that we shall not err, as 
