THE KEY. STEPHEN J. PEKliY ON THE MAGNETIC 
Table IX. 
Station. 
Computed T. F. 
Observed T. F. 
Obs. — Comp. 
(Error) 2 . 
Paris 
100686 
10-0676 
-00010 
Laval 
10-0670 
10-0668 
-0-0002 
0-00000004 
Brest 
10-1352 
10-1228 
-0 0124 
000015376 
Vannes 
10-0706 
100802 
+00096 
000009216 
Angers 
10 0301 
10-0196 
-0-0105 
0-00011025 
Poitiers 
9-9672 
9-9701 
+0 0029 
0 00000841 
Bordeaux 
9-8816 
9-8487 
-0 0329 
O 00108241 
Abbadia 
9-8162 
9-8351 
+0-0189 
000035721 
Bayonne 
9-8172 
9-8626 
+0-0454 
0002061 16 
Pau 
9-7913 
97539 
-0-0374 
000139876 
Toulouse 
9-7S42 
9-7818 
-0-0024 
000000576 
Perigueux 
9-8834 
9-8887 
+0-0053 
0-00002809 
Bourges 
9-9682 
9-9713 
+0-0031 
000000961 
Amiens 
10-1292 
10-1403 
+0-0111 
0-00012321 
0 2 ]= 0-00543083 
The most probable error of any one observation will therefore be 
=r=0-6745^/l^ = 0-0144. 
For the sake of comparison with the results of former observations, and in particular 
of those of Dr. Lamont, we will determine the direction of the lines of equal intensity 
for the horizontal component of the earth’s magnetism. 
By a process identical with that employed for the Total Force, we now find that at 
our central station the Horizontal Force=4T156, 
x= -0-00037605, ^=0-0013401, and r= 0-0013919, and «= — 74° 19' SOM; 
or the direction sought is 
N. 74° 19' SOM E. to S. 74° 19' SOM W. ; 
the probable error in any single observation being now O’ 006 7. 
We will next place side by side the values of the Horizontal Force for 1858 and 1868. 
I 
Table X. 
Station. 
Jan. 1st, 1858. 
Sept. 1st, 1868. 
Difference. 
Amiens 
3-9639 
40129 . 
+00490 
Angers 
4-1450 
4-2088 
+00638 
Bayonne 
4-4875 
4 5504 
+0-0628 
Bordeaux 
4-3730 
4-4093 
+0-0363 
Paris 
4-0685 
41116 
+0-0431 
Perigueux 
4-3699 
4-4252 
+00553 
Poitiers 
4-2341 
4-2938 
+00597 
Toulouse 
4-5243 
4-5867 
+0-0624 
Hence the yearly increase of the Horizontal Force in the West of France =0-0050. 
Dr. Lamont gives 0-0048 as the value for 1858, therefore we may conclude that there 
is a slight acceleration of about 0-00002 per annum in the increase of this element. 
