SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 
87 
attraction, and a single pair of observations made at so short an interval as a year is 
hardly sufficient to rely on. The results from Stonyhurst, however, prove that the 
assumed and actual rates of change may differ by 0'’8, and that, thus. Dips reduced 
to January 1, 1886, from the end of 1888, may be about 2''5 wrong. To prevent, as 
far as possible, any effect on the general run of the isoclinal lines from this uncer¬ 
tainty, we put down on a map all the assumed rates at places for which they could be 
determined, and took the rate at any other place to be that given by the nearest of 
these stations. 
The rates of decrease thus obtained for the south of England are considerably less 
than those assumed by M. Moureaux for the north of France, which do not differ 
much from — 2'7. These values are obtained for the interval 1858-1885 by com¬ 
paring M. Moureaux’s observations with Lamont’s map. We think, however, that the 
observations of the Rev. S. J. Perky, F.R.S., reduced to the epoch, January 1, 1869, 
prove that in France as in England the rate of change of Dip has been diminishing. 
Twelve stations were common to Father Perry and M. Moureaux; at one of 
these, Marseilles, the station of the earlier observer was no doubt seriously affected 
by local disturbance. The rate of change for the epoch 1858-69 is much larger than 
for any other station, and for 1869-85 it is much too small. Rejecting this observa¬ 
tion we think the following table shows that M. Moureaux’s numbers are too large. 
Secular Change of Dip in France. 
Station. 
Jan. 1, 1869. 
Jan. 1, 1885. 
Secular variation. 
1869-85. 
1859-85. 
Amiens. 
66 40-3 
6°6 80 
- 2-0 
- 2-7 
Bordeaux .... 
63 23-0 
62 41-8 
- 2-6 
- 3T 
Clermont .... 
63 36-4 
62 52T 
- 2-8 
- 2-8 
Dijon. 
64 24-5 
63 53-4 
- 1-9 
- 2-7 
Grenoble .... 
62 54-2 
62 6’9 
- 2-9 
- 2-6 
Monaco. 
61 22T 
60 41-2 
- 2-5 
- 2-8 
Moulins .... 
64 4-9 
63 30T 
- 2-2 
- 2-7 
Paris. 
65 52'5 
65 17-3 
- 2-2 
- 2-8 i 
Perigueux .... 
63 23-9 
62 44-9 
- 2-4 
- 3T 
Poitiers .... 
64 28T 
63 55-6 
- 2-0 
- 2-8 1 
Toulouse .... 
62 IT 
61 23-9 
- 2'3 
- 3-2 
Mean .... 
— 2'35 
- 2-85 i 
We turn next to the secular change of the Declination. 
A number of stations are common to our own survey and those of 1837 and 1857, 
and we have also observed at several places where the Declination was determined in 
1872. The mean annual decrease calculated for the observations is exhibited in the 
following table :— 
