12 
THE REV. STEPHEN J. PERRY ON THE MAGNETIC 
No. 3 at Avignon, and No. 2 at Clermont, and I find that these arbitrary exclusions do 
not tend to improve the results. It is, however, a different case with regard to the two 
stations of Marseilles and Grenoble, where we were unable to procure very convenient 
sites for the observations. Omitting, therefore, these two stations in our equations of 
condition, we obtain 
£=65-7658, r=0-0108, u=- 74° 10' 13"-56, 
with +0-06550 as the probable error at any single station, the probable error of the 
mean being +0-01544. This diminution in the probable errors would seem to warrant 
the omissions. 
Considering the limited time at our disposal we were unable in this survey of France 
to choose many stations at which Dr. Lamont had previously observed ; but this want of 
identity of locality may be balanced by a comparison of the general results obtained 
from all the observations made during the two surveys. Employing precisely the same 
method to reduce Lamont’s values for 1858 as has been used above, we arrive at the 
following results : — 
Table V. 
Epoch. 
Dip at 
Central Station. 
Dist. of isoclinals 
differing by 0°'5. 
Angle of isoclinals 
N.E. of meridian. 
Number of 
observations. 
Jan. 1, 1858, W 
66-6291 
miles. 
40-36 
70 23 25 
16 
Jan. 1, 1858, E. 
66-4640 
44-44 
72 44 33 
15 
Sept. 1, 1868, W 
65-8796 
43-84 
73 32 50 
13 
Sept. 1, 1869, E 
65-7816 
44-80 
75 14 34 
20 
We thus obtain 0°-0703 as the annual variation of the Dip in the west of France, 
whilst in the east it only varies annually 0-0585 ; and the isoclinals appear to be 
receding much more rapidly from the meridians in the west than in the east. 
The Magnetic Intensity. 
We next proceed to discuss the observations for determining the lines of equal 
intensity. 
