Royal Society of Edinburgh. 103 
occur in tliis movement, but seems to belong to the second we have 
mentioned, viz., that of mastication. 
The articulating surface, strictly speaking, on each condyle appears 
to constitute the thread, or rather part of the thread, of a conical 
screw passing over an axis lying at or about right angles to the 
plane of motion in simple opening and closing of the jaws. This 
spiral course of the articular surface is perhaps best seen in some of 
the larger carnivora, such as the lion, but is also obvious in a well- 
developed human condyle. 
The action of this conical screw or tap within the glenoid cavity, 
considered as the conical die, takes place with accuracy only when 
one joint alone acts with the condyle within the glenoid cavity— the 
other condyle being beyond it, and gliding upon the surface of the 
zygoma, as during mastication. The food is in this process crushed 
between the molar teeth of that side whose condyle remains within 
the glenoid cavity ; this condyle screwing the jaw back, so to speak, 
to its natural position at each closure of the teeth. 
By this construction a great amount of friction is avoided ; what 
would otherwise be a rubbing being thus converted into a rolling 
motion between the condyloid and glenoid surfaces ; while by one or 
other condyle always remaining in the glenoid cavity during masti- 
cation greater steadiness and security is afforded to the joint. 
5. On some properties of Ice near its Melting Point. By- 
Professor Forbes. 
“ During the last month of March I made some experiments on the 
properties of ice near its melting point, with particular reference to 
those of Mr Faraday, published in the Athenseum and Literary 
Gazette for June 1850, to which attention has been more lately 
called by Dr Tyndall and Mr Huxley in relation to the phenomena 
of glaciers. 
“ Owing to indisposition, I have been obliged to leave my experi- 
ments for the present incomplete. But I am desirous, before the 
session of the Boyal Society closes, to place on record some facts 
which I have observed, and also some conclusions which I deduce 
from these and other recent experiments and discussions. 
“ Mr Faraday’s chief fact, to which the term ‘ regelation’ has 
been more lately applied, is this, that pieces of ice, in a medium 
