SURVEY OE BELGIUM IN 1871. 
349 
Table XI. 
Station. 
Observed T. F. 
Computed T. F. 
Observed — Computed. 
Aix-la-Chapelle 
10-1064 
10 1129 
-00065 
Alost 
10-2049 
100976 
+01073 
Antwerp 
10-0606 
101064 
-0-0458 
Arlon 
10-0896 
100917 
-00021 
Bruges 
100355 
10-0958 
-00603 
Courtray 
101139 
10-0886 
+0-0253 
Ghent 
101268 
10-0974 
+0-0294 
Liege 
10 0563 
10-1064 
-00501 
Louvain 
10-0873 
10-1024 
-0-0151 
Mons 
100804 
10-0882 
-0-0078 
Namur 
10-0349 
100965 
-0-0616 
Ostend 
101112 
10-0927 
+00185 
Spa 
1015/0 
10-1052 
+0-0518 
Tournay 
100813 
100859 
-0-0046 
Tronchiennes 
101430 
10-0966 
+00464 
Turnhout 
10-1/08 
101125 
+00583 
The probable error at any station would thus be + 0 - 0321, whilst that of the mean is 
+ 0-0080. 
If we examine carefully the above figures, we find that the intensity increases as we 
proceed Northwards, but that the computed and observed values do not agree as to the 
Eastward or Westward tendency of the lines. The observed values in the West are 
greater than those in the East ; but when the distance of each station from the meridian 
of Brussels is taken into account, the isodynamics are found to lie N.W. and S.E. This 
result is confirmed by combining the observed values by the method of least squares, 
which gives 10-10525 as the Total Force at the central station, and for the direction of 
the lines N. 70° 3' 34 ,f, 6 W. to S. 70° 3' 34"-6 E. That this is not caused by mere 
accidental errors at certain stations may be clearly shown ; for, casting out those 
stations whose values are abnormal, and then recalculating the position of the lines of 
equal Intensity, we obtain a very similar result, even when as many as the six least 
reliable values are omitted. It is unfortunate that Lamont’s survey of 1858 does not 
afford sufficient data for computing the isodynamics in Belgium alone for that epoch ; 
but if we take his stations in Belgium and Holland, along with the neighbouring posi- 
tions of Aix-la-Chapelle and Emden, we obtain nine equations of condition, which furnish 
lines of equal Intensity common to the two countries, and the direction of these is from 
N. 57° 6' 6"-3 E. to S. 57° 6' 6" -3 W. The probable errors are very nearly the same as 
those given above, being +0-031582 for a single station and +0-010527 for the mean. 
Of the three stations common to the two surveys the error for Aix-la-Chapelle is positive, 
whilst those of Ghent and Mechlin are negative. We have therefore indications of the 
existence of some great disturbing cause, which, by increasing the intensity at the 
eastern stations, tends to alter the direction of the isodynamics, and to remove further 
apart the lines passing through points whose Total Force differs by a constant quantity. 
A centre of local magnetic force may therefore not unnaturally be sought for in the 
eastern portion of Belgium, and the geological map of the country presents at once a 
ready answer to the inquiry. Starting from the neighbourhood of Mons, the vast coal 
tracks of Belgium, rich in iron ore, stretch across the eastern provinces, enveloping 
3 A 
MDCCCLXXIIT. 
