TRANSACTIONS OF THE SECTIONS. 39. 
also towards the pole, but is only a thin stratum forming part of a secondary circu= 
lation ;—that the middle current moves from the pole, and constitutes the return cur- 
rent for both the preceding ;—and that all these three currents have a prevailing 
motion from west to east in advance of the earth. This is the substance of Mr. 
Thomson’s theory ; and he gives, as an illustration, the following simple experiment: 
—If a shallow circular vessel with flat bottom, be filled to a moderate depth with 
water, and if a few small objects, very little heavier than water, and suitable for 
indicating to the eye the motions of the water in the bottom*, be put in, and if the 
water he set to revolve by being stirred round, then, on the process of stirring being 
terminated, and the water being left to itself, the small particles in the bottom will 
be seen to collect in the centre. They are evidently carried there by a current deter- 
mined towards the centre along the bottom in consequence of the centrifugal force of 
the lowest stratum of the water being diminished in reference to the strata above 
through a diminution of velecity of rotation in the lowest stratum by friction on the 
bottom. The particles being heavier than the water, must, in respect of their den- 
sity, have more centrifugal force than the water immediately in contact with them ; 
and must therefore in this respect have a tendency to fly outwards from the centre, 
but the flow of water towards the centre overcomes this tendency and carries them 
inwards; and thus is the flow of water towards the centre in the stratum in contact 
with the bottom palpably manifested. 
On the Plasticity of Ice. By James Tuomson, C.E. &c. 
Mr. Thomson commenced by stating, that to Prof. James Forbes is to be attributed 
the discovery that the motion of glaciers down their valleys depends on a plastic or 
viscous quality of the ice. He (Mr. Thomson) had formed a theory to explain the 
nature of this plasticity, and the manner in which it originates. He had been led to 
his speculations on this subject from a previous theoretical deduction at which he 
had arrived, namely, that the freezing-point of water, or the melting-point of ice, 
must vary with the pressure to which the water or the ice is subjected, the tempera- 
ture of the freezing-point being lowered as the pressure is increased. His theory on 
that matter} led to the conclusion that the lowering of the freezing-point for one 
additional atmosphere of pressure must be 00075 Centigrade, and that the lowering 
of the freezing-point corresponding to other pressures must be proportional to the 
additional pressure above one atmosphere. The phenomena which he thus predicted, 
in anticipation of direct observations, were afterwards fully established by experi- 
ments made by his brother, Prof. William Thomson, of which an account was pub- 
_ lished in the ‘ Proceedings of the Royal Society of Edinburgh for February 1850.’ 
_ Having thus laid down as a basis the principle of the lowering of the freezing-point 
_ of water by pressure, Mr. Thomson proceeded to offer his explanation, derived from 
_ it, of the plasticity of ice at the freezing-point, as follows :—If to a mass of ice at 
0° Centigrade, which may be supposed, for the present, to be slightly porous, and to 
contain small quantities of liquid water diffused through its substance, forces tend- 
_ ing to change its form be applied, whatever portions of it may thereby be subjected 
to compression will instantly have their melting-point lowered so as to be below 
_ their existing temperature of 0° Centigrade. Melting of those portions will therefore 
set in throughout their substance, and this will be accompanied by a fall of tempera- 
_ ture in them, on account of the cold evolved in the liquefaction. The liquefied por- 
tions being subject to squeezing of the compressed mass in which they originate, will 
spread themselves out through the pores of the general mass, by dispersion from the 
regions of greatest to those of least fluid pressure. Thus the fluid pressure is relieved 
_in those portions in which the compression and liquefaction of the ice had set in, ac- 
companied by the lowering of temperature. On the removal of this cause of liquidity, 
the fluid pressure, namely, the cold, which had been evolved in the compressed parts 
* A few tea-leaves taken from a teapot will suit the purpose well. 
+ This theory is to be found in a paper by the author, entitled ‘‘ Theoretical Considera- 
_ tions on the Effect of Pressure in lowering the Freezing-point of Water,” published in the 
Transactions of the Royal Society of Edinburgh, vol. xiv. part 5, 1849, and also re-published 
_ in the Cambridge and Dublin Mathematical Journal for Nov. 1850, vol. v. p. 248. 
& . 
