SURVEY OF THE WEST OF FRANCE. 
37 
Table IV. 
Station. 
Computed Dip. 
Observed Dip. 
Obs. — Comp. 
(Error) 2 . 
Paris 
65 52-44 
65 53-75 
+ 131 
Laval 
65 46 02 
65 49 5 
+3-48 
12 1104 
Brest 
66 33-96 
66 27-8 
-616 
37-9456 
Yannes 
65 45-24 
65 48-4 
+3-16 
9-9856 
Angers 
65 1716 
65 9 7 
-7-46 
55 65 16 
Poitiers 
64 28-80 
64 29-4 
+0-60 
0-3600 
Bordeaux 
63 19-32 
63 24 2 
+4-88 
23 8141 
Abbadia 
62 25 38 
62 29 0 
+362 
13-1044 
Bayonne 
62 27 42 
62 31*4 
+3-98 
ln-8404 
Pau 
62 7-98 
61 59-4 
-8-58 
73-6104 
Toulouse 
62 5 -46 
62 22 
-3-26 
10 6276 
Perigueux 
63 22-92 
63 25-0 
+2 08 
4-3264 
Bourges 
64 33 00 
64 33-7 
+0-70 
0-4900 
Amiens 
66 40-44 
66 41-3 
+ 0-86 
0-7396 
(V] =258-61 21 
The most probable error for any single observation is then given by the formula 
where ^ = 0 , G745, and m=13; Probable error =3'T3. 
The disturbing influence from geological causes may be judged of by Table V., which 
is drawn up from the map of MM. Dufrenoy and Elie de Beaumont, Xngenieurs des 
Mines. 
Table V. 
Station. 
Error. 
Geological nature of the position. 
Pau 
8-58 
Terrain tertiaire superieur. 
Angers 
7-46 
Terrain de transition. 
Brest 
616 
Terrain crystallise, primitif ; gneiss. 
Bordeaux 
4-88 
Depots posterieurs aux dernieres dislocations du sol. Terr. tert. sup. 
Bayonne 
3-98 
Terrain cretace inferieur. 
Abbadia 
3 62 
Laval 
3-48 
Terrain de transition. 
Toulouse 
326 
Depots posterieurs. 
Yannes 
316 
Terrain crystallise, primitif. 
Perigueux . . 
208 
Terrain cretace inferieur. 
Paris 
1-31 
Terrains tertiaires inferieurs. 
Amiens 
086 
Depots posterieurs. 
Bourges 
0-70 
Terrain Jurassique, systeme oolitique. 
Poitiers 
0-60 
” 
The error at Pau was most probably clue to the fine dust that filled the air, the obser- 
vations having been taken not far from a building in the course of erection. The amount 
of error at Angers and at Brest might also partly be attributed to the less favourable 
situation in which the instruments were used. Most of the stations were, however, well 
adapted to magnetic observations, being on sedimentary rocks or later deposits, and 
equally free from all disturbing influences. 
In constructing a map of the isoclinal lines from the above values of u and r, it must 
be borne in mind that the assumptions of these lines being straight, of their parallelism, 
or of a uniform rate of increase of the dip, are only first approximations to the truth , in 
so extensive a tract of country as that covered by the Survey. The curvature of the 
lines towards the north as we approach the west should be very considerable. 
The secular variation of the dip has not been taken into account in the above calcu- 
