SURVEY OF THE EAST OF FRANCE. 
23 
Table XVIII. 
Station. 
Declination, 
Jan. 1, 1858. 
Declination, 
Sept. 1, 1869. 
Difference 
of Epoch. 
Difference of 
Declination. 
Yearly rate 
of change. 
Declination, 
Jan. 1, 1869. 
Clermont 
18-568 
16-342 
Hf 
- 2*226 
— 0-191 
16-460 
Dijon 
17-932 
16-503 
-1-429 
-0-122 
16-612 
Marseilles 
17-068 
15-579 
-1-489 
-0-128 
15-691 
Moulins 
18-653 
16-368 
-2-285 
-0-196 
16-487 
Paris 
19-605 
17-140 
>> 
-2-465 
-0-211 
17-260 
Mean 
— 0-1696 
This result is somewhat larger than that found for the west of France, which was 
— 0°T533 ; and the greater difference between the results obtained from the observations 
at the several stations makes the result less trustworthy. 
I will next proceed to collect in a single Table the chief results connected with the 
Isogonics of the surveys of 1858 and 1868-69. 
Table XIX. 
Epoch. 
Declination at 
Central Station. 
Distance of 
Isogonics differing 
by 0°'5. 
Angle of Isogonics 
N.E. of geographic 
meridian. 
Number of 
observations. 
Jan. 1, 1858 W. ...... 
19-6390 
miles. 
50-4 
22 18 24-9 
18 
Jan. 1, 1858 E. 
19-6053 
46-0 
16 37 16-9 
17 
Sept. 1, 1868 W 
17-9498 
44-2 
21 41 0-9 
13 
Sept. 1, 1869 E 
17-4493 
60-2 
17 2 30 
18 
This gives as the secular decrease of the Declination in 
the west and east 0T583 and 0-1848 respectively, which are 
somewhat in excess of the values found from the few stations 
common to both the surveys. 
The secular diminution of the Dip and Declination, and 
the increase of the Horizontal Force, in both the east and 
west of France, so clearly indicate the actual position of the 
North magnetic pole, together with its motion round the 
extremity of the earth’s axis of rotation, that we are led to 
examine whether this same motion of the pole may not also 
account, at least in part, for the differences observable in 
the results obtained from the east and west surveys. 
In the annexed diagram let P represent the geographical 
pole, W and E the two portions of the country surveyed, 
and p, p' the positions of the magnetic pole corresponding 
to the epochs 1858 and 1869. 
If X stands for the magnetic latitude, and § for the Dip, 
we shall have tan ^ = 2 tan X ; and consequently the change 
