SURVEY OF BELGIUM IN 1871. 
347 
Table VII. 
Station. 
H. F. 
Y. F. 
T. F. 
m. 
j Aix-la-Cliapelle 
4-0045 
9-2792 
10-1064 
0-46052 
1 Alost 
3-9503 
9-4054 
10-2049 
0-45887 
Antwerp 
30274 
9 2623 
100606 
0-46031 
Arlon 
4-1157 
9-2121 
10-0896 
0-45991 
Bruges 
3-8934 
9-2495 
10 0355 
0-45949 
Brussels Observatory ... 
3-9595 
9-3252 
10-1310 
0-45956 
Brussels, rue Koyaie ... 
3-9189 
9-2899 
10-0827 
0-45948 
Courtray 
40011 
9-2889 
10.1139 
0-45999 
Ghent 
3-9180 
9-3382 
10-1268 
0-45892 
Liege 
40126 
9 2211 
10-0563 
0-46024 
Louvain 
3-9544 
9-2799 
100873 
0-46073 
Mons 
4-0048 
9-2507 
10-0804 
0-45897 
Namur 
3-9923 
9-2066 
10 0349 
0-46052 
Ostend 
3-9136 
9-3226 
101112 
0-45881 
Spa 
40221 
9-3267 
101570 
0-45976 
Tournay 
3 9958 
9-2556 
100813 
0-45889 
Tronchiennes 
3-9016 
9-3626 
101430 
0-45896 
Turnhout 
3 9522 
9-3715 
10-1708 
0-46062 
The second members of our original equations remaining each unchanged, but being- 
three fewer in number, Table VII. will enable us at once to form the equations of con- 
dition for determining the nature of the lines of equal Horizontal Force. Their solution 
leads to the following values: H. F. =: 3-96649, x— —0-00030302, ?/=(M)015808. 
Hence we find that there is a distance of 62-13 miles between the lines, indicating a 
difference of 0-1 in the Horizontal Force, and that this system of lines is inclined at an 
angle of 79° 8' 55" - 7 to the meridian, the direction being from N. 79° 8' 55"-7 E. to 
S. 79° 8' 55"-7 W. The differences between the observed results and those computed 
from the above data furnish a Table for calculating the probable errors. 
Table VIII. 
Station. 
Observed. 
Computed. 
Obs. — Comp. 
Aix-la-Chapelle 
4-0045 
3-9988 
+0 0057 
Alost 
39503 
3-9529 
-0 0026 
Antwerp 
39274 
3-9268 
+0-0006 
Arlon 
41157 
4 1138 
+00019 
Bruges 
3-8934 
3 9126 
-0-0192 
Courtray 
4-0011 
3 9562 
+00449 
Ghent 
3-9180 
3-9363 
-00183 
Liege 
40126 
4-0050 
+0-0076 
Louvain 
3-9544 
3-9668 
-0-0124 
Mons 
40048 
40052 
-00004 
Namur 
3-9923 
40153 
-00230 
Ostend 
3-9136 
3-9064 
+0-0072 
Spa 
4 0221 
4 0267 
-00046 
Tournay 
39958 
3-9807 
+00151 
Tronchiennes 
3-9016 
3-9355 
-0 0339 
Turnhout 
3-9522 
3-9230 
+00292 
This gives us as the probable error for the mean at a single station +0 -01 327, whilst 
that at the central station is +0-00332. 
In order to determine the secular change of the Horizontal Intensity, I have collected 
together, and reduced to English units, all the previous observations with which I am 
acquainted. The values for 1858 are taken from Dr. Lamont’s ‘ Erdmagnetismus,’ and 
those for 1839 and 1854 may be found scattered in the introduction to the same work. 
