58 
ARCHDEACON PRATT ON THE ATTRACTION OF THE 
In the second place, I propose to reduce the formula to numbers, and so arrive at 
such an approximate value of the attraction as the data I have been able to collect 
will allow. 
12. This approximate value is, as will be seen, larger than 5"-236, the error brought 
to light by the Survey. I make various suppositions with a view, if possible, to 
reduce my result to this, but without effect. This leads me to look in another direc- 
tion for an explanation of the cause of discordance, and I arrive at a conclusion 
which clears up the discrepancy, confirms the calculations of this paper, and illustrates 
the importance of not disregarding the influence of mountain attraction. 
I. Determination of a Formula for calculating Mountain Attraction on the 
stations of the Indian Arc. 
13. Let O be the centre of a circle AQ, AT the 
tangent at A, QR a slender prism of mass M, being 
the prolongation of the radius through the point Q. 
Then if AQ=a, AK—h, Z QAR=(y, and AOQ=^, 
the following is true : — 
Lemma. — The attraction of the prism QR on the 
point A in the direction AT 
For let P be any point of the prism, QP= 2 , QR=A, 
I PAQ=^t, 
dz 
.'. mass of element of prism at P=M-^j 
Fig. 2. 
o 
attraction of this on A in direction AP 
1 
PA^’ 
/• I • A • 1- • -XU COS PAT 
attraction of this on A in direction A1 = M — PA^’ 
Cos PAT=cos 4^) 
a 
1 , 
cos - 3 
cos 
sin CO 
’ K’ 
cos -3 
= QP=fl — - = fl("cosi3tan [^3 + 4/) -sin ^ 3^ 
cos (43 + 4/) ^ ^ / - / 
dz 1 
^^_ttCOS 2 
