SURVEY OP THE EAST OP FRANCE. 
19 
Table XIII. 
Epoch. 
Intensity at Central 
Station. 
Distance between 
Isodvnamics which 
differ by O'l. 
Angle of Isodynamics 
N.E. of meridian. 
Number of stations. 
Jan, 1, 1858, W. 
10-1951 
miles. 
91-7 
63 52 40-6 
16 
Jan. 1, 1858, E.... 
10-2000 
96-9 
60 18 43-7 
15 
Sept. 1, 1868, W. 
10-0688 
113-9 
70 39 17-8 
13 
Sept. 1, 1869, E. 
10-0608 
113-8 
66 55 24-9 
20 
It is evident from these figures that the variations of the Dip and Horizontal Force 
combine to produce a very rapid alteration of the isodynamics, especially by increasing 
the distance between the lines. 
The largeness of the error in the Total Force at Marseilles and Grenoble warrants a 
recalculation of the results with the omission of these two stations. The following are 
the values obtained by this reduction : — 
T.F. = 10-0566, #=0-0003253, y= -0-0007253, r=0-000795, u= 65° 50' 40"-8. 
The probable errors are thus very much diminished, being now only 0-01306 for a 
single station, and for the mean 0-00308,. 
The Magnetic Declination. 
The determination of this magnetie element, which at a fixed observatory presents 
but little difficulty, is by far the most troublesome and the least to be relied upon when 
the observations have to be taken in the course of a magnetic survey. For not only 
must the magnetic instruments themselves be in perfect condition, as for the other 
observations, but any unknown change of rate in the chronometer, any error in the 
determination of the sun’s position, is sufficient to introduce a serious inaccuracy in the 
results, to say nothing of the perturbations so much more frequent and more extensive 
in this element than in the others. 
The Frodsham chronometer used during this survey has given perfect satisfaction, its 
rate having been remarkably constant during the whole journey, even more so than in 
18G8. This will be seen from the following comparisons : — 
Table XIV. 
Station. 
Date. 
G. M. T. 
Error. 
Daily rate. 
Stonyhurst Observatory 
j, ...... 
5> 
Paris Observatory 
Marseilles Observatory 
Paris Observatory 
Stonyhurst Observatory 
» 
)> 
July 20 
„ 21 
„ 25 
Aug. 7 
27 
Sept. 14 
„ 25 
Oct. 9 
» 24 
h m s 
9 21 5-5 p.m. 
9 51 25-0 p.m. 
9 57 10-0 p.m. 
10 20 A.M. 
9 43 A.M. 
1 0 P.M. 
7 15 40-5 P.M. 
6 54 45-0 p.m. 
7 15 15-5 p.m. 
m s 
+ 5 4-77 
+ 5 6-77 
+ 5 15-71 
+ 5 39-65 
+ 6 18-97 
+ 6 54-70 
+ 7 13-30 
+ 7 34-58 
+ 7 57-27 
s 
+ 2-00 
+ 2-24 
+ 1-84 
+ 1-966 
+ 1-985 
+ 1-69 ' 
+ 1-52 
+ 1-51 
d 2 
