SURVEY OF THE WEST OF FRANCE. 
35 
are herein omitted for brevity’s sake. The fullest inquiries have invariably been made as 
to the exact position of any underground iron pipes that might interfere with the obser- 
vations. 
In the following calculation of the most probable dip at each station, I have omitted 
the observations taken at Vannes with needle No. 3, since these were made during heavy 
rain and under a lofty but covered ball court. The readings of No. 3 at Amiens have 
also been excluded, since they were only taken as a verification of the results obtained 
with No. 1, and were only single readings. 
The latitudes and longitudes in Table II. are taken from the 4 Connaissance des Temps,’ 
and the differences between the latitude and longitude of Paris, and those of the other 
stations, have been calculated in geographical miles by aid of the Table in Loomis’s 
4 Practical Astronomy,’ and each verified by measurement on the geological map of 
MM. Dufrenoy et Elie de Beaumont. 
Table II. 
Station. 
Latitude. 
Longitude. 
Difference of 
Latitude. 
Difference of 
Longitude. 
Paris 
48 50 11 
h m s 
0 0 0 
geogr. miles. 
geogr. miles. 
Laval 
48 4 7 
12 27 w. 
- 53 
+ 143 
Brest 
48 23 32 
27 19 
- 31 
+318 
Yannes 
47 39 30 
20 23 
- 81 
+ 235 
Angers 
47 28 17 
11 34 
- 94 
+ 134 
Poitiers 
4(5 34 55 
7 59 
-156 
+ 93 
Bordeaux 
44 50 19 
11 40 
-276 
+ 138 
Abbadia 
43 23 7 
16 21 
-376 
+ 196 
Bayonne 
43 29 29 
15 16 
-369 
+ 183 
Pau 
43 17 44 
10 51 
-383 
+ 130 
Toulouse 
43 36 33 
3 35 
-361 
+ 43 
Perigueux 
45 11 4 
6 28 
-252 
+ 76 
Bourges 
47 4 59 
15 E. 
-121 
- 3 
Amiens 
49 53 43 
8 w. 
+ 73 
+ 2 
For the latitude and longitude of the Magnetic Observatory at Abbadia I am indebted 
to the kindness of M. d’Abbadie, Membre de l’lnstitut, who rendered us every assistance 
during our stay in the South of France. The pillar on which most of the observations 
at this station were taken stands at about 800 yards N. of the Astronomical Observatory. 
In forming the equations of condition, by which the most probable value of the dip 
at each station can be determined by the method of least squares, I have chosen Paris 
as the origin of coordinates for several reasons. The chief of these arises from the fact 
that Paris is practically the centre of France, and thus observations could easily be made 
there at the beginning and end of the Survey of 1868, and also of 1869. Add to this 
that frequent observations have in past times been made there, and that the able staff 
under the direction of M. Le Yerrier keep up a continued series of determinations of 
the magnetic elements. Lastly, the nature of the soil guarantees a perfect freedom from 
the disturbing influence of igneous rocks, &c. 
