Prof. John Mitne—On the Cooling of the Earth. 99 
The most important points of difference between Onycopyge and 
Deiphon are observable in the thorax or middle-body, the pleura in 
the former genus being firmly articulated to one another at the 
fuleral points, whereas in the latter they are ungrooved and free. 
The pygidium in Deiphon is short, with a minute axis of four joints, 
and is quite truncated below; the pygidium in Onycopyge is well 
developed, and has indications of seven coalesced somites. Onycopyge 
apparently possessed nine free thoracic somites; Deiphon has ten 
somites. 
With Staurocephalus it agrees in the more compact and firmly 
articulated character of the thoracic somites ; in Onycopyge, however, 
the extremities of the somites lengthen perceptibly from the head to 
the pygidium ; whereas in Staurocephalus the anterior pleure are the 
longest. 
In Staurocephalus the genal portion of the head is not reduced to 
a mere spine (as is the case in Deiphon and Onycopyge), but has well- 
developed fixed cheeks. The pygidium in Staurocephalus is composed 
of four minute coalesced somites ; and when provided with prominent 
spines it has, like the genus Deiphon, one spine only on each side. 
It is to be hoped that this interesting Trilobite is only the precursor 
to many others which Professor Liversidge may have the good 
fortune to obtain, and I may have the pleasure to describe, from 
the Australian Continent. 
II.—Nore Upon tue Cootinc oF THE EARTH. 
By Prof. J. Mitnz, F.G.S., 
Imperial College of Engineering, Tokei, Yedo. 
1 eae every text-book on geology speaks of the heat which has 
been lost by the earth, and the consequent contractions which 
have taken place upon its surface. The way in which all this has 
come about, however, is either not explained at all or else very 
imperfectly, and the difficulty experienced by any one endeavouring 
to find out what the conditions have been will be fully recognized by 
reference to the lengthy correspondence which was carried on in the 
columns of Nature (December, 1878, and January, 1879) between Mr. 
A. R. Wallace, Mr. O. Fisher, and others, upon this subject. A fair 
idea of the succession of conditions which occur in a cooling sphere 
like our earth may be obtained by combining together the results 
given by Sir William Thomson in his writings on the secular cooling 
of the earth (see Thomson and Tait’s Natural Philosophy, vol. i. pp. 
711-727), with the results given by Professors Perry and Ayrton in 
a paper entitled, ‘‘ Experiments on the Heat Conductivity of Stone” 
(Phil. Mag. April, 1878). In this latter paper we have given, and 
I believe for the first time, a series of curves which show the rate of 
cooling of a ball at its various parts. Two of these curves are repro- 
duced in the accompanying figure, Fig. 1. PA represents the cooling 
at the centre, and PF the cooling at the circumference, the direction 
OP measuring temperature, and the direction OX measuring time. 
Curves for intermediate points between the centre and the circum- 
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