18 
T HE EEY. STEPHEN J. PERRY ON THE MAGNETIC 
magnetism at the central station is 10 '0608. The probable errors can now be deduced 
by a comparison of the values computed from the above data with the intensity at each 
station, found by combining the observations of the Dip and Horizontal Force. 
Table XI. 
Station. 
From observed 
Dip and H. F. 
Computed. 
Error. 
Rheims 
10-0907 
10-0577 
+ 0-0330 
Metz 
9-9958 
10-0171 
-0-0213 
Strasburg 
9-9346 
9-9608 
-0-0262 
Xssenheim 
9-9528 
9-9362 
+ 0-0166 
Mont Eolland 
9-9632 
9-9152 
+ 0-0480 
Dole 
9-9251 
9-9125 
+ 0-0126 
Dijon 
9-9361 
9-9322 
+ 0-0039 
Lyons ............ 
9-8780 
9-8381 
+ 0-0399 
Avignon 
9-7897 
9-7360 
+ 0-0537 
Marseilles 
9-6048 
9-6993 
-0-0945 
Monaco 
9-7142 
9-6884 
+ 0-0258 
Montpellier 
9-7461 
9-7425 
+ 0-0036 
Grenoble 
9-7252 
9-7999 
-0-0747 
N. D. de Myans 
9-8236 
9-8149 
+ 0-0087 
Mongre 
9-8796 
9-8622 
+ 0-0174 
St. Etienne 
9*8417 
9-8364 
+ 0-0053 
Clermont 
9-8956 
9-8778 
+ 0-0178 
Moulins 
9-9134 
9-9174 
-0-0040 
Douay 
10-1226 
10-1351 
-0-0125 
Boulogne 
10-1431 
10-1777 
-0-0346 
Paris 
10-0551 
10-0608 
-0-0057 
These errors, omitting that for Paris, since it is not included in our equations, give 
as the probable error at any single station ±0’0253, whilst that for the mean is 
±0-00566. 
The stations common to the surveys of 1858 and of 1868-69 will furnish us with the 
data for calculating the secular changes of terrestrial magnetic intensity. 
Table XII. 
Station. 
T. F., Jan. 1, 1858. 
T. F., Sept. 1, 1869. 
Difference of epoch. 
Difference of T. F. 
Yearly rate of 
change. 
Clermont 
10-0007 
9-8956 
Hf 
— 0-1051 
-0-0090 
Dijou 
9*9979 
9-9361 
5 ? 
— 0-0618 
-0-0053 
Marseilles ...... 
9-7649 
9-6048 
» 
-0-1601 
-0-0137 
Montpellier ... 
9-8355 
9-7461 
5 ? 
— 0-0894 
-0-0077 
Moulins 
10-0356 
9-9134 
-0-1222 
-0-0105 
Paris 
10-1793 
10-0551 
— 0-1242 
-0-0106 
Mean 
-0-00947 
The secular variation deduced from the general results of all the observations is con- 
siderably larger than the above, being 0-0118 for the west and 0-0119 for the east of 
France. These are obtained from the subjoined Table. 
