614 
MR. A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
Thus it is often possible to follow for some distance a line of maximum Vertical 
Disturbino- Force, on which the values of the Vertical Disturbances are either 
absolutely small or but little exceed tliose at neighbouring stations. Such lines may 
exert comparatively little eftect on the Horizontal Disturbing Forces, but they are 
often found to connect regions of high Vertical Force, which wmdd otherwise appear 
entirely isolated, and to explain apparent anomalies in the directions of the Horizontal 
Forces. The magnetic phenomena which they exhibit are such as would be produced 
by vertical dyke-like sheets of magnetic matter, of which the horizontal thickness 
was small, compared with either the horizontal length or the depth. Such vertical 
laminae would produce greater effects upon the Vertical than upon the Horizontal 
Force. The two vertical faces woidd be oppositely magnetised and would largely 
neutralize each other’s effects. The lower horizontal face being narrow might be 
neglected. The upper horizontal face would, therefore, be the most important. For 
the purpose of illustration let it be regarded as an infinite straight line at a depth d 
below the surface. The Vertical and Horizontal Disturbing Forces would be equal 
and the latter would be a maximum at points on the surface at a distance d from 
that vertica.lly above the line. Let them at that point be equal to O’OOOSO C.G.S. 
unit, i.e., to the limit of the accurate determination of the direction of the Horizontal 
Disturbance. 
Under these conditions the Horizontal Disturbance ^rould everywhere else be less 
than that limit, and thus the disturbing cause would not produce any clearl}^ marked 
effect on the Horizontal Forces. On the other hand, throughout a strip, the breadth 
of which was ecpial to twice the depth of the disturbing line, the Vertical Dis¬ 
turbance would vary between O'OOOdO and O'OOOGO C.G.S. unit. The larger of these 
quantities is nearly double, the smaller is about equal to the average error of the 
determinations of the Vertical Force Disturbance. 
Hence the effect of the line on the Vertical Force would be detected, while the 
Disturbance of the Horizontal Force would probably escape notice. The result would 
be still more likely to follow if the line ran through a region subject to other 
disturbances, due to more distant but moi'e powerful centres of attraction. These 
might give to the Horizontal Forces directions altogether independent of the minor 
j’idge, the existence of which might nevertheless be traced by a clearly marked, 
though comparatively unimportant line of relative maxima of the Vertical Dis¬ 
turbances. Such lines may be regarded as secondary ridge lines. They connect all 
the principal magnetic districts, and generally so clearl}- and simply that we cannot 
but believe that they are the less conspicuous parts of the magnetic system, of which 
the more prominent features were discovered in our earlier work. 
The plan which we have adopted in working up our results has been as follows:—- 
First the ridge and valley lines were drawn in Map 12, as far as possible by the aid 
of the Vertical Disturbing Forces alone. The lines of maximum and minimum 
Disturbance are often so clearly marked that no possible doubt can arise. Sometimes, 
but comparatively seldom, it was necessary to consult the Horizontal Disturbing 
