46 
THE REV. STEPHEN J. PERRY ON THE MAGNETIC 
An application of the method of least squares, similar in all details to that made use 
of above, furnishes the equations 
20T25=13D-fl690a;+2480y, &c. 
Hence 
D=0*92921, tf=0-01056199, y= -0-0039534 ; 
; , =0°'0112T8 per geographical mile, i.e. the rate of change in the declination along 
the normal to the isogonic lines is 1° for every 88 *7 geographical miles, and u= — 20 31' 16", 
or the direction of the isogonic lines is 
N. 20° 31' 16" E. to S. 20° 31' 16" W. 
We might naturally expect that observations of this magnetic element would lead to 
much less satisfactory results than those of the Dip or Total Force ; for the almost con- 
tinuous, and often energetic, action of disturbing forces render any limited number of 
absolute readings of the declination at different stations a very uncertain guide in calcu- 
lating the exact direction taken by the isogonic lines ; whereas in the case of the vertical 
and horizontal components of the total force the daily disturbances are of a much less 
exaggerated character, and the maximum perturbations of not frequent occurrence. The 
subjoined Table will show the probable amount of error at each station. 
Table XIY. 
Station. 
Computed Declination. 
Observed Declination. 
Diff. 
Paris 
17-929 
1 7°-891 
- 2-28 
Laval 
19-251 
19-129 
- 7-32 
Brest 
21165 
21-059 
- 6-36 
Vannes 
20-091 
20-279 
+ 11-28 
Angers 
18-973 
19-146 
+ 10-38 
Poitiers 
18-295 
18-358 
+ 3-78 
Bordeaux 
18-296 
18-259 
- 2 22 
Abbadia 
18-513 
18-285 
-13-68 
Bayonne 
18-403 
18-439 
+ 2-16 
Pau 
17’788 
J 7-873 
+ 5-10 
Toulouse 
17-044 
17167 
+ 12-66 
Perigueux 
17-736 
17-726 
- 0-60 
Bourges 
17-419 
17-047 
-22-32 
Amiens 
18-239 
18-358 
+ 7 14 
The probable error of any single observation will therefore be 6'' 954 9. We are now 
able to determine the secular variation of the declination. 
Table XV. 
Station. 
Epoch January 1st, 1858. 
September 1st, 1868. 
Variation. 
Amiens 
o / // 
19 56 IS 
o / // 
18 21 27 8 
— 1 34 50-2 
Angers 
20 16 18 
19 8 44 0 
- 1 7 34 0 
Bayonne 
19 57 48 
18 26 20-8 
-1 31 27-2 
Bordeaux 
20 0 12 
18 15 33-8 
-1 44 38-2 
Paris 
19 36 18 
17 53 27-8 
-1 42 50 2 
Perigueux 
19 26 30 
17 43 34-3 
-1 42 55-7 
Poitiers 
19 56 24 
18 21 27-1 
-1 34 56 9 
Toulouse 
18 45 0 
17 10 0-7 
-1 34 59-3 
