The Magnetism of Basalt. 45 
will be most readily seen in the table in the case of the bars 
by the Cambridge Instrument Co. and by Messrs. Elliott 
Bros. In the former case the bending increased the distance 
by almost exactly 1 part in 10,000 at all distances. 
It is not at all improbable that the method or modifications 
of it might be found useful in a variety of elasticity measure- 
ments, especially when made on non-magnetic materials. 
The deflecting magnet might be replaced by a coil traversed 
by an electric current. It would be easy to carry a small 
coil or light magnet at the end of an arm, so as to bring it 
very close to the magnet or suspended coil meant to be 
deflected. In this way one could arrange that an elastic 
displacement, largely multiplied if desired, should produce a 
large effect on the field of a galvanometer-needle. Readings 
of the deflexion-angle of a calibrated galvanometer, or of the 
movement of a compensating coil situated at some distance, 
and movable by a fine screw so as to secure a balance, might 
afford an exceedingly sensitive method. 


VI. On the Magnetism of Basalt and the Magnetic Behaviour 
of Basaltic Bars when Heated in Arr, By G. BE. Auuay, 
D.Sc. (Birm.), late Lecturer in Physics in the University 
of Birmingham*. 

[Plates I. & IL] 
= the magnetism of rocks has an important bearing 
not only from the geological but also from the physical 
standpoint, viz. in its relations to regional magnetic distur- 
bances, has been shown by Prof. Sir A. Riicker +, in a 
paper on this subject, in the first part of which are given 
determinations of the susceptibility of a large number of 
different kinds of rock, basaltic and others. The magnetic 
properties of the numerous ferriferous minerals have also 
been the object of many investigations f. 
The subject matter of the present paper is confined chiefly 
to one kind of rock, viz. basalt ; and the experiments de- 
scribed were made in order to find at what temperature basalt 
becomes non-magnetic. The effect of temperature on mag- 
netite is, to a large extent, already known, and as the 
magnetism of basalt is taken to be due chiefly to the magnetite 
it contains, it might be assumed that the magnetic properties 
of this rock are always similar to, while feebler than, those 
* Communicated by the Physical Society: read October 23, 1903, 
+ A. W. Riicker, Proc. Roy. Soc. vol. xlviii. ie 505 (1890). 
t Congrés Int. de Physique, Rapports 2, p. 470 (1900). 
