684 
ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
8. A mass of ice was placed on a flat rough slab of sandstone, so arranged that it 
could easily be placed at any proposed inclination to the horizon. When the inclina- 
tion was about 20°, the ice descended with an accelerated motion as in ordinary cases, 
but at smaller inclinations it descended with a slow uniform motion, which, for inclina- 
tions not exceeding 9° or 10°, was, cwteris paribus, proportional to the inclination. 
The velocity was increased by an increased weight of the mass, the area in contact with 
the plane remaining the same. The motion was due to the melting of the ice in contact 
with the slab, for when the temperature of the air became below that of freezing, it 
entirely ceased. 
The motion was sensible for inclinations little exceeding half a degree, and, doubtless, 
would also have been so for smaller inclinations. This shows how small a force was 
required to move the mass when its lower surface was in a state of disintegration. Let 
f be a retarding force applied to a mass whose weight =W, placed on a plane whose 
inclination is /. Then will W sin i—fhe the moving force along the plane; and since 
the mass will require only the smallest force to move it, a force f very nearly equal to 
W' sin i will be necessary to hold the mass at rest. If f be removed, the ice will begin 
to move with a moving force nearly = W sin /, the motion being permitted by the hque- 
faction of successive indefinitely thin layers of ice, but then it will be retarded by the 
solid mass coming in contact with the surface on which it slides. A melting of another 
indefinitely thin layer will then take place, and the above process will be repeated, the 
velocity increasing till the continuous action of the plane on the mass becomes equal to 
the weight resolved in the direction of the plane. During this time (probably too short 
to be estimated) the motion will be an accelerated one, but will thenceforward become 
uniform, the action of the plane becoming equal to the resolved part of the weight along 
it. The uniform velocity is, in fact, a terminal velocity, similar to that of a stone 
descending in water, when it soon approximates to a nearly uniform motion. The 
action of the inclined plane on the moving mass, like that of the resistance of the water 
on the stone, has this property — that, while it is incapable of exerting any but the 
smallest force to hold the body absolutely at rest, it exerts a retarding force upon it in 
uniform motion, equal to that of gravity. The objections above mentioned against the 
sliding of glaciers have arisen from an entire misconception of this kind of mechanical 
action. 
9. At the period when the preceding experiment was published, I was disposed to 
think that the greater part of the observed motion of the surface of a glacier was due 
to the general motion, by sliding, of the whole mass, while it was contended by other 
glacialists that it was principally due to an excess of the velocity of the upper strata 
of the mass over that of its lower strata, due to a gradual change of form of the whole 
mass, and that there might in fact be no sliding movement at all. In recognizing that 
both these causes might be verce causae*, I urged the necessity of deciding on their relative 
claims by actual observations, which should determine the velocities of the upper and 
lower surfaces of a glacier at some point where the lower one was accessible. Observa- 
* Philosoptical Magazine and Journal for February 1845. 
