SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1886. 323 
The determination of the rate of secular change is a more difficult matter, but at 
the completion of our work we have more data at our disposal than in the earlier 
stages, when coefficients had to be chosen for the reduction of observations. The 
plans adopted in the case of each element are as follows :— 
1. Declination. 
We have deduced from Sir F. Evans’s Map for 1872 (J,oc. cit.) the Declinations 
at 24 points distributed uniformly all over the kingdom, and have compared them 
with the values given by our Table VI11., on j^age 238. The secular corrections thus 
calculated shovv' an increase from east to west amounting to about O'Hl and 0'’14 per 
degree of longitude in latitudes 58° and 52° respectively. 
There is also an increase with latitude above the latitude 52°. 
The results are exhibited in the folio wins: Table :— 
Table XI.—Mean Secular Change of Declination per annum between 1872 and 1886. 
Latitude. 
Longitude. 
10° w. 
8° W. 
6° W. 
4° W. 
2° W. 
0°. 
0 
60 
/ 
! 
/ 
9-1 
9h 
' 
58 
9-6 
9-2 
8-8 
8-9 
56 
9-1 
8-6 
8-3 
8-2 
54 
9-3 
8-5 
8-1 
7-8 
7-8 
7-9 
52 
9-4 
8-9 
81 
7-8 
7-8 
7-7 
50 
8-3 
8-0 
This is in fair accord with the observations at individual stations given on p. 88, 
but as Sir Frederick’s isomagnetics were not deduced by any definite system of calcu¬ 
lation, but little importance must be attached to minor variations and discrepancies. 
We do not, for instance, feel justified in assuming that there is a real increase of 
the secular coefficient to the south of lat. 52°, especially as M. Moureaux’s values in 
France for corresponding intervals are less than ours. 
It is also in accord with the results of the comparison of our observations with 
those of Welsh, which show that between 1857 -1886 the secular change in Scotland 
increases with the latitude and with wmst longitude. We think, therefore, that the 
most that can be said at present is that the secular change at the time of our survey, 
as given by the mean of the Greenwich and Kew values (p. 91), was in the neigh¬ 
bourhood of London about 6'‘5 per annum, and that a comparison with Sir F. Evans’s 
results shows that it is about l'‘5 greater in the south-west of Ireland, and about 
2' larger in the north-east of Scotland. 
Hence by smoothing the irregularities in Table XI. above, and reducing all the 
numbers so as to give the present rate at London we get the figures in Table XII.. 
p. 325. 
2 T 2 
