SURVEY OE THE WEST OE FRANCE. 
41 
Table VIII. 
Station. 
Horizontal Force. 
Vertical Force. 
Total Force. 
Moment of Magnet. 
Paris 
4-1 1(52 
9-1926 
100721 
0-4852 
Laval 
41 220 
91840 
100668 
0-4845 
Brest- 
4-0424 
9-2807 
10-1228 
0-4852 
Vannes 
41310 
9-1948 
10-0802 
0-4854 
Angers 
4-20SS 
90926 
100196 
0-4851 
Poitiers 
4-2938 
8-9979 
9-9701 
0-4820 
Bordeaux ... 
4-4093 
8-8064 
9-8487 
0-4834 
Abbadia 
4-5439 
8-7225 
9-8351 
0-4820 
Bayonne 
4-5504 
S-7498 
9-8626 
0-4823 
Pau 
4-5807 
8-6113 
9-7539 
0-4819 
Toulouse . . . 
4-5867 
8-6397 
9-7818 
0-4833 
Perigueux ... 
4-4252 
8-8432 
9-8887 
0-4831 
Bourges 
4-2830 
9-0045 
9-9713 
0-4828 
Paris 
4- 1070 
9-1869 
100631 
0-4828 
Amiens 
4-0129 
9-3128 
101403 
0 4823 
Deflections not having been observed at Bonrges, it was necessary to compute the 
value of to, in order to eliminate it from the product toX obtained by the vibrations. 
The mean value of to during the survey was 0*48335 at epoch Sept. 1st, 1868; the 
mean for the remainder of 1868 was 0*48037, therefore the variation per month 
298 
= H= = 119*2, .*. the value on Sept. 14 should have been 0-48283. This agrees well 
with the mean value of to at Perigueux on the 12th and Paris on the 16th, i. e. 0-48294. 
Forming, now, our equations of condition from the above data, and solving them by the 
method of least squares, precisely in the same manner as in the case of the dip, we ob- 
tain the equations 
12-3419=13/+1690tf+2480y, &c., 
where /= the total force diminished by 9. 
From these we obtain 
6279-2889 = -59137/+239731870+ 
4089465*7980=3699700/+ 469679480^- ; 
.*. 977426255=914711558/ and /=l-06857. 
This gives for the most probable computed value of the total force at Paris 10*0686, 
whereas the mean of the direct observations was 10-0676 ; difference =0 - 0010. 
By substitution we at once get the quantities 
a*=0-00028979,y= -0-0008223; .*. r=0*000S72, and u=- 70° 34' 13"*1. 
Hence the direction of the isodynamic lines is 
N. 70° 34' 13"*1 E. to S. 70° 34' 13"*1 W., 
and the total force changes 0-1 for every 115 geographical miles along the normal to 
these lines. 
e can now draw up a Table of the computed errors in the observations. 
MDCCCLXX. 
G 
