734 
ME. HOPKINS ON THE THEOET OF THE MOTION OF GLACIEES. 
The pressure in any tube would cause no reflux of the water, or other disturbance in a 
confluent one, because the pressures in them would be equal ; but it must be carefully 
observed that there would, in this case, be no internal fluid pressure greater than that 
of the atmosphere, and therefore none capable of producing any efiect in expanding 
the general mass, and urging it onwards. But if, on the contrary (as in the second case 
above explained), the tube be such that the pressure in each tube considered separately 
should be greater than the condition of an equality of pressure at the point of con- 
fluence of any two tubes would not be generally satisfied, and the whole motion would 
be interfered with. If the pressure p in any number of tubes became great, it would 
manifestly produce a reflux in any confluent tube in which the pressure was smaller, and 
a consequent outpouring of water from every pore and crevice on the surface of the 
lower regions of a glacier. Such, in fact, must necessarily be the case wherever there 
is an internal fluid pressure much exceeding that of the atmosphere, and where all parts 
of the mass are permeated by ducts of any kind through which water can flow out of 
the mass with little impediment. 
Hence it appears that if the internal ducts were such as not to impede the cmTents 
passing through them by means of friction, or any kind of capillarity, there might indeed 
be large internal pressures, but they would necessarily be accompanied by overflowings 
from the superficial pores and crevices in many parts of a glacier. But such a pheno- 
menon would be entirely opposed to all observation ; and it is this which constitutes the 
proof that there can be no great fluid pressure in the interior of a glacier due to water 
contained within it in tubes which exert no sensible retarding efiect by friction on the 
water running through them. This conclusion agrees with that expressed by Principal 
Foebes in the last of the preceding extracts; but the reason assigned is very difierent. 
The absence of great fluid pressure within the glacier in such case is not due to the 
absence of capillarity, but to the particular condition of its ducts and crevices being open 
in every part of the bounding surface of the mass. There can be no expenditure of vis 
viva in producing pressure on a smooth surface, as is well known, though such pressm*e 
must necessarily be produced in the case before us, as in all cases of constrained motion 
when the constraint is produced by the action of such surfaces. The fact of no vis viva 
being lost does not imply that no internal pressure would be produced. 
62. But let us now turn to the far more important case in which the passage of the 
water through the small ducts of any mass, as that of a glacier, is impeded by the 
friction of the sides of the ducts. It follows, as shown in the preceding explanation, 
that, since in actual cases there is no overflowing of water from the pores and crevices, 
here assumed to pervade one part of the mass as well as another, the fluid pressm'e in 
all confluent ducts at the point of confluence must be the same, and, therefore, inde- 
pendent of the height above that point at which each duct may meet the external 
surface of the glacier. This pressure, when the surface is exposed to the atmosphere, 
can be only the atmospheric pressure. Hence it follows in this case, as in the previous 
one, that there can be no internal fluid pressure greater than that of the surrounding 
