640 
MR. A. W. RtiCKER AXD DR. T. E. THORPE OX A MAGNETIC 
from the edge of the dyke; the Horizontal Disturbance would be below the limit of 
the accurate determination of direction at a distance of a little over 450 feet. 
If an observer, seeking for the dyke by the aid of the magnet, missed the central 
line by 200 yards, he would not detect the presence of the igneous rocks. 
In general the Vertical Faces would also be magnetized, but they would affect the 
distribution of the Force near the dyke rather than extend the limits of its influence. 
Thus if we suppose them to be truly vertical and perpendicular to the magnetic 
meridian, the Vertical Disturbing Force will be positive on the southern and negative 
on the northern side. 
The Horizontal Disturbance will be negative [i.e., it will act southwards) above 
and to the north of the dyke. 
The following table gives the magnitudes of these forces, the columns being 
arranged as before :— 
o 
Distance in 
feet. 
Disturbing 
Forces. 
Horizontal. 
Vertical. 
0 
-no 
0 
.50 
- 67 
± 70 
150 
± 3 
± 33 
Adding these algebraically to the numbers previously obtained, we get the following 
table :— 
Distance in 
feet. 
Disturbing Forces. 
Horizontal. 
Vertical. 
150 S 
101 
43 
50 S 
141 
272 
0 
-no 
329 
50 N 
-275 
132 
150 X 
- 95 
- 23 
These figures prove that under the assumed conditions the forces due to the 
Vertical Faces do not produce important effects, except close to the dyke Itself. 
It is, perhaps, needless to give similar tables to illustrate the case of a nearly 
Horizontal Dyke. The upper and lower faces are oppositely magnetized, and they 
neutralize each other’s effects except within a short distance of the edge. It may at 
first sight seem extraordinary that a sheet so widely extended as that which is 
indicated by the outcrop of the Whin Sill should produce no far-reaching etfects, but 
it would be cpiite contrary to the theory we are discussing if such results were 
observed. 
