356 
THE KEY. STEPHEN J. PEEEY ON THE MAGNETIC 
combined with the fact that the errors of the observed Dip at both stations are positive 
and their errors of H. F. both negative, might justify the assumption that the disturbing 
cause is situated at no great distance from the observatory, and probably at some depth 
and to the W. of the N. meridian. The probable errors of the observed Dip and H. F. 
at the observatory were not very large, being respectively +7' 15" and —0*0070. 
It may be of advantage, in particular for the discussion of the errors of the Total 
Force, to present here in a tabular form a geological description of the stations of 
observation. The data in this Table are taken from the excellent map of M. Andre 
Dumont, a copy of which was kindly presented to me for this purpose by his son. 
Table XVIII. 
Station. 
Aix-la-Chapelle. 
j Alost 
Antwerp ... 
j Arlon 
i Bruges 
1 Brussels .. 
I Court ray 
Ghent 
Liege 
Lierro 
Louvain.. 
Mechlin . 
Mons .... 
Namur . 
Ostend . 
Spa 
Tournay. 
Tronchiennes. 
Turnhout 
Verviers 
Geological formation. 
Terrain cretace, systeme Aachenien. E.S.E. houille. 
Terrain tertiaire, eocene, glaueonie. 
„ ,, pliocene, systeme Scaklisien. 
,, jurassique, systeme liasique. 
,, tertiaire, miocene, argile verte et, sable marin. 
,, „ eocene, glaueonie superieure. 
„ ,, ,, glaueonie ; sable et gres de Beauchamp. 
„ ,, ,, glaueonie moyenne. 
„ ,, ,, glaueonie superieure. 
E., W., N. houille. S. terrain anthraxifere, systeme Eifelien, quartzoschistcux. 
Terrain tertiaire, pliocene, systeme Diestien. 
„ ,, N. miocene, argile verte et sable marin ; S. glaueonie superieure ; E., W. 
pliocene, systeme Diestien. 
„ „ miocene. argile verte, sable marin, et gres de Fontainebleau. 
E., N.E., S.W. houille. S. terrain tertiaire. eocene, glaueonie moyenne. 
Terrain anthraxifere. N. systeme Condrusien, calcareux ; S. houille. 
Alluvions. 
Terrain Ardennais. N.W. terrain Rhenan, systeme Gedinnien. 
„ tertiaire, eocene, glaueonie. S.E. terrain anthraxifere, systeme Condrusien, cal- 
careux. 
Terrain tertiaire, eocene, glaueonie superieure. 
,, „ miocene, systeme Bolderien. 
„ Ardennais, systeme Salmien ; et terrain Rhenan, systeme Gedinnien. 
The three maps annexed to this paper (Plates XIX.-XXI.) show the general direction 
of the lines of equal Dip, H. F., and Declination. The broken lines for 1871 are calcu- 
lated on the supposition of the isoclinals, isodynamics, and isogonics being approxi- 
mately straight, and the observed mean values are attached to each station in order to 
show how nearly these straight lines represent the actual curves. 
This has also been made more evident by introducing the curves of equal values, 
which are represented by continuous lines. The method adopted in drawing these lines 
is shown in the map of the isodynamics ; and the shaded portions of the same map indi- 
cate the position and extent of the coal-beds of Belgium, which lie, for the most part, 
along the banks of the Meuse. 
The greatness of the accidental errors, as w y ell as the narrow limits of the tract of 
country under consideration, renders it very difficult to trace, with any great degree of 
accuracy, the probable curvature of the actual lines of equal Dip, Declination, and Inten- 
sity. But bearing in mind that in all cases the errors arising from defects of station or 
of observation mask to a great extent the real nature of these curves, we may still clearly 
