PRESENT VARIATION OF THE COMPASS. 
321 
abstract as the secular change. These corrections, though not strictly accurate, as will 
be hereafter seen, from the unequal values of the annual changes of the magnetic decli- 
nation on the several coasts of the United Kingdom, are nevertheless far within the 
limits of the probable errors of the observations themselves, and may therefore, I 
apprehend, be safely adopted as bringing the several observations forward to one 
common epoch, as well as clearing them from diurnal and other inequalities. 
The results thus brought to one epoch were placed on a Mercator’s Chart of the 
British Islands, and the lines of equal declination for each degree graphically drawn 
through the several values. In this delineation of the lines of equal value I was greatly 
assisted, both in the determination of their direction and the slight amount of necessary 
curvature, by numerous observations of the magnetic declination made in neighbouring 
countries and in the adjacent seas extending to the Arctic Ocean, which cannot be intro- 
duced in the appended Chart, but which had been used in the preparation of a Magnetic 
Variation Chart of the World, published by the Admiralty in 1871. 
The procedure here adopted is, under the conditions of so extended an area and limited 
number of observations, perhaps better adapted for a truthful representation of the Iso- 
gonic lines than their calculation according to the usual method, where the differences 
of the values of the magnetic elements are linear functions of the differences of latitude 
and longitude, and accordingly straight lines. 
From the records of the fixed Magnetic Observatories in this and adjacent countries, 
it is now certain that the annual change, i. e. a decrease of westerly declination, is gra- 
dually accelerating, and in some localities notably so within the last ten or fifteen years : — 
At Brussels*, between 1850 and 1860, the mean annual decrease was 5'*38 ; between 
1860 and 1868 it was 8 ,- 17. 
In Norway, at Christianiaf, 
between 1850 and 1855 the annual decrease= 8'45 
1855 „ 1860 „ = 9-48 
1860 „ 1865 „ =10-52 
At Paris J, 
between 1825 and 1858 „ = 5 - 0 
1858 „ 1868-7 „ = 9-6 
f 
The annual change of the Westerly Declination at the well-known Magnetic Obser- 
* See ‘Notices Extraites cle l’Annuaire de l’Observatoire Royal de Bruxelles’ for 1869, by the Director, 
A. Qtjetelet, from which the following is extracted for the purpose of the text : — 
Annual 
p o r change. 
1850, 12th April, 10| a.h., observed W. Declination 20 25-71 
1860, 4th „ 1| p.m. „ „ 19 31-9 1 ~ 
1868, 1st „ Noon „ „ 18 26-5/ 65 ' 4==8 ‘ 17 
t See ‘Notices Extraites de l’Annuaire de l’Observatoire Royal de Bruxelles’ for 1864, in a letter from 
M. Hansteen to M. A. Qtjetelet. 
X See Phil. Traus. for 1870, p. 47, “ Magnetic Survey of West of France,” by the Rev. S. J. Perry. 
2x2 
