SURVEY OF BELGIUM IX 1871. 
okp; 
odd 
ascertain the actual yearly change of this element. Dr. Lamont observed at Aix-la- 
Chapelle, Ghent, and Mechlin ; but as I did not take the Declination at Mechlin, the 
comparison is confined to two stations. 
Table XVII. 
Station. 
nation, 
Jan. 1, 1858. 
Declination, 
Sept. 1, 1871. 
Secular 
variation. 
18-243 
16-544 
0-124 
Ghent 
19-560 
17-983 
-0115 
Mean f 
or epoch 1864-8 ... 
-01195 
If we use the corrected values for September 1, 1871, the secular variation becomes 
— 0T255. Lamont gives — 0T267 for 1858. 
From the long series of observations made at the Royal Observatory of Brussels by 
MM. A. and E. Quetelet, from 1828 to 1871,1 find the following values of the secular 
variation : — 
Epoch 1838-75 .... -0-0896 
„ 1849-5 .... -0-1065 
„ 1860-25 .... -0-1234. 
The yearly acceleration is therefore constant, and = — 0-00158. 
The observations at Brussels give very unsatisfactory results, when we regard the 
absolute values, and not merely the yearly change at a permanent unaltered station. In 
1858 Lamont found a difference of 28' between the Declination at the observatory and 
that at a station not far distant ; E. Quetelet confirmed this result the same year, the 
difference being 30'; and now, at positions so close together as the observatory and 131 
rue Royale, I find a still greater mean difference. The Declination observations made 
at Brussels during this survey were as follows : — 
Uncorrected values. 
Corrected values. 
Computed results .... 
. 17-3881 
17-3368 
Observatory, August . . . 
. 18-0716 
18-0072 
,, September . 
. 17-8931 
17-9195 
5? If 
. 18-0775 
18-0775 
,, Error of mean . 
+ 0-6260 
-f 0-6646 
131 rue Royale, August . 
. 17-1334 
17-2125 
,, September . 
. 17-4261 
17-3850 
„ Error of mean 
- 0-1083 
- 0-0380 
The station near the observatory, though apparently not free from disturbing 
influences, gives from the corrected values a result almost identical with the computed 
quantity, since it is within the range of its probable error, whilst the result for the 
observatory itself is more than half a degree in excess. This difference of Declination, 
