646 
MR. A. W. RtiCKER DR. T. E. THORPE OX A MAGNETIC 
The average of 86 specimens of basalt and gabbro from the West of Scotland and 
Ireland was 0'00245. Using these values we get the following results ;— 
Horizontal Disturbing Force. 
A*. 
Less than 2-8*.-. 
Possibly not greater 
tbanOlK-. 
■ 1 
0-00160 
0-00448 
■ 
0-00018 
0-00215 
0-00686 
0-00027 
In other words, whereas experiment showed that the Force was < 0'00400, 
calculation proves that, on the most extravagant assumptions, it could not possibly 
have exceeded that amount by more than 50 per cent., while it may have been 20, 
and very possibly was 10 times less. 
There is, therefore, nothino- in the result obtained at Canna to throw doubt on 
calculations based on the assumption that regional magnetic Disturbances are due to 
masses of magnetic rock. 
It may, however, very properly be urged that tlie observation at Canna was made 
at sea, and that although the Disturbance could not be detected by the compass, it 
would have been easily measured if the magnetometer had been placed in the same 
position on good non-magnetic observing ground. 
This is quite true, and on the island itself the Disturbances were very great, but 
the statement that such effects die out quickly with distance, is supported by obser¬ 
vations made elsewhere on land. 
It may be well to exhibit in a table the Forces due to a rectangular mass of basalt, 
of which the top and bottom surfaces are squares and the sides are vertical, Avhen it 
is magnetized by the earth’s magnetic field in the United Kingdom. 
We will suppose the mass to be so arranged that two sides are in the magnetic 
meridian. The other two are, therefore, magnetized by the horizontal component of 
the earth’s field. 
The point, for which the Disturbances are calculated, is in the vertical plane which 
bisects these two sides, in the plane of the upper surface. All lengths are expressed 
in terms of that of a horizontal side of the parallelopipedon. 
The Forces are calculated as before for the twm susceptibilities 0'0016 and 0'0024. 
The Disturbances are in terms of O'OOOOl C.G.S., i.e., O'OOOl metric unit. 
The results are as follows :— 
