Notices of Memoirs—Prof. Hull on Terrestrial Magnetism. 327 
Instances of polarity in basaltic masses at various localities were 
adduced in order to illustrate the possibility of polarity in the in- 
ternal mass. Magnetic polarity had been found by the author to 
exist in segments of basaltic columns. 
The subject of the polarity of the globe was then discussed, and it 
was pointed out how the position of the “magnetic poles” leads to 
the inference that they are in some way dependent upon the position 
of the terrestrial poles. 
The author regarded the so-called “double poles” as merely foci 
of attraction due to protuberances of the magnetic magma into the 
exterior non-magnetic magma, and maintained that there was really 
only a single magnetic pole in each hemisphere, embracing the 
whole region round the terrestrial pole and the stronger and weaker 
magnetic foci, and roughly included within the latitude of 70° within 
the northern hemisphere. 
- It was pointed out that the poles of a bar-magnet embrace a com- 
paratively large area of its surface, and hence a natural terrestrial 
magnet of the size here hypothecated may be inferred to embrace 
a proportionably large tract for its poles. 
In reference to the question why the magnetic poles are situated 
near those of the earth itself, this phenomenon seemed to be con- 
nected with the original consolidation of the crust of the globe, and 
the formation of its internal magmas. 
It was suggested that, owing to the differences of temperature 
which must have existed in the polar regions, as compared with 
those of the equatorial, the process of solidification may have been 
more rapid in the polar regions than elsewhere, and it was inferred 
that in the case of the magnetic magma the process of crystallization 
and the polar arrangement of the particles of magnetic iron-ore might 
have proceeded from the poles towards the equator in radial direc- 
tions. The manner in which the phenomena of magnetic intensity, 
and of the dip of the needle at different latitudes, could be explained 
on the hypothesis of an earth’s internal magnet, such as is here 
described, was then pointed out; and the analogy of such a magnet 
with a magnetic bar passing through the centre of the earth was 
illustrated. On the other hand it was conceivable that the process 
of solidification might have commenced at the Equator—extending 
towards the poles, and thus the polar arrangement would naturally 
arise. 
The author then proceeded to account on geo-dynamical principles 
for the secular variation of the magnetic needle, and also endeavoured 
to show how the objections that might be raised to the views here 
advanced, on the grounds of the high temperature which must be 
assumed to exist at the depth beneath the surface of the magnetic 
magma, could be met by considerations of pressure; and on this subject 
read a letter which he had received from Sir William Thomson, F.R.S. 
