10 
THE KEY. STEPHEN J. PERRY ON THE MAGNETIC 
3-461=5— 114 #+197 y 
3-026=5 — 100 #+235 y 
3- 571=5- 36 #+211 y 
4- 045=5— 48 #+157 y 
6- 746 = 5- 33 #—106 y 
7- 087=5+ 32 #-131 y. 
These equations of condition combine to form the three simultaneous equations, — 
75-454= 205- 2521#+ 3057y, 
— 8544-730 = — 25215 + 421413#— 447498y, 
6571-180= 30575— 447498# + 910595y, 
which give as the most probable values of the three unknowns — 
5= 5-7816, 
#= 0-0028495, 
y = — 0-0107928. 
Thus the mean value of the Dip at the central station is 65°-7816 ; whilst the distance 
between the isoclinals that differ by 30' is 44-8 miles, r being =0 o, 01116 ; and the angle 
formed by the isoclinals with the meridian is — 75° 14' 34", i. e. their direction is from 
N. 75° 14' 34" E. to S. 75° 14' 34" W. 
The substitution of the above values of 5, #, and y in the equations of condition forms 
the Table by which we can determine the most probable error in a single observation 
or in the mean. 
Table III. 
Observed Dip. 
Computed Dip. 
Error. 
Rheims 
65-903 
65-876 
+ 0-027 
Metz 
65-425 
65-502 
-0-077 
Strasburg 
64-553 
64-883 
-0-230 
Tssenheim 
64-567 
64-527 
+ 0-040 
Mont Rolland 
64-225 
64 097 
+ 0-128 
Dole 
64-178 
64-062 
+ 0-116 
Dijon 
64-373 
64-287 
+ 0-086 
Lyons 
63-233 
63-152 
+ 0-081 
Avignon 
61-806 
61-795 
+ 0-011 
Marseilles 
60*540 
61-223 
-0-683 
Monaco 
61-332 
61-256 
+ 0-076 
Montpellier 
61-578 
61-667 
-0-089 
Grenoble 
62-867 
62-597 
+ 0-270 
N. D. de Myans 
62-838 
62-796 
+ 0-042 
Mongre 
63-461 
63-331 
+ 0-130 
St. Etienne 
63-026 
62-961 
+ 0-065 
Clermont 
63-571 
63-403 
+ 0-168 
Moulins 
64-045 
63-951 
+ 0-094 
Douay 
66-746 
66-832 
— 0-086 
Boulogne 
67-087 
67*287 
-0-200 
