AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 
83 
of that which I designed and tested in the Irish Magnetic Survey of 1915 for 
horizontal forces. With such instruments more accurate observations can be 
obtained, and moreover, it might be possible to observe about six stations per day 
instead of one as at present. 
PAET II.—ON THE MAGNETIC SUSCEPTIBILITY OF CERTAIN 
ROCK SPECIMENS. 
By Prof. Ernest Wilson, King's College, London. 
The magnetic permeability of a substance is given by the ratio of B, the 
magnetic induction, to H the magnetising force. The magnetic susceptibility k is 
related to the permeability n by the expression 
/X = 1 + Itt/v. 
The susceptibility as above defined refers to unit volume, and is sometimes called the 
volume-susceptibility to distinguish it from the mass-susceptibility obtained by 
dividing it by the density. 
A special instrument was devised capable of measuring quickly susceptibilities of 
a low order of magnitude, and certain of the results so obtained were compared with 
those obtained by the ordinary magnetometer method. 
For use with the new instrument of balance the specimens have a length of 4 cm. 
or less, and in most cases an approximately circular cross-section of 1 cm. diameter. 
In other cases, the cross-section has been approximately square and 1 cm. in the 
side. For the magnetometer tests the specimens had a length of about 10 cm. and 
approximately square cross-section of 1'5 cm. in the side. When using the balance 
the magnetic force is applied transversely, and with the magnetometer it is applied 
longitudinally. In the first case the magnetic force in C.G.S. units (H) varies from 
about 6'7 to 300 ; and in the second the variation is from about 33 to 100. The 
susceptibility of any one specimen has been found to be practically constant between 
these limits. 
The following table* gives the description of and locality of the specimens examined. 
The volume susceptibility of each specimen is given in C.G.S. units, together with 
the corresponding magnetising force H employed at the time of test. 
In one or two cases where confirmation seemed desirable, specimens have been 
re-tested ; for example, the basalt E. 6520.* The specimen L. 63 as originally tested 
* In the table and throughout this paper L. denotes a specimen collected in connection with the 
Leicestershire observations; N. one in connection with the Northamptonshire observations; E. specimens 
belonging to the collection of the Geological Survey at the Museum of Practical Geology. 
M 2 
