92 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
creases regularly as the depth increases. Thus, from the velocity of 
the current corresponding to a wave 150 feet high in a sea of the 
depth of 600 feet, we may conclude that the velocity of the current 
for a wave of 100 feet high in the same sea would be about 14 miles 
an hour; and in the same way we may conclude, from the results 
given in the table for the velocities of the currents attending waves 
of 200, 300, and 400 feet respectively, that the velocity for a wave of 
the same height in a sea of 500 feet deep would be about 73 miles 
an hour. 
I have before remarked that these results must be considered only 
as approximative, but they prove beyond all doubt that paroxysmal 
elevations, beneath the sea, varying from 50 to 100 feet in height, may 
produce currents of which the velocities shall vary from at least 5 or 
6 to 15 or 20 miles an hour, provided the depth of the sea do not 
exceed 800 or 1000 feet. 
We have next to consider the magnitude of the blocks which might 
thus be moved. 
23. The magnitude of the force which a given current can exert 
on bodies of certain forms entirely immersed in the fluid, has been 
determined by numerous and satisfactory experiments, as well as the 
law according to which the force varies with the velocity of the 
current. The force exerted on a surface given in magnitude and po- 
sition, is found to increase as the square of the velocity, up to the 
greatest velocities which have been experimented on, about 9 or 10 
miles an hour ; and the same may doubtless be extended by mduc- 
tion to much greater velocities. A curious consequence results from 
this law, when we estimate the force of the current (as we are natu- 
rally led to do in the case before us) by the weight of the largest 
block of a given form which it is capable of transporting. Thus 
estimated, the force varies as the szxth power of the velocity of the 
current. 'Thus, a certain current being able to move a cube of given 
weight, another current of double the Velocity would move a cube of 
64 times the weight of the former ; if the velocity were treble that 
of the first case, the weight of the cube which could be moved by it 
would be 729 times as great, and so on. This is the result of the 
simplest calculation, and shows how mistaken an estimate we might 
form of the motive power of currents of great velocity, from the con- 
sideration of that of ordinary streams. 
The magnitude of the block which may be moved by a given 
current depends much upon its form, those forms which approach 
nearest to the spherical being most favourable. It also may depend, 
in certain cases, on the depth of the water, the most favourable bemg 
that which should not be greater than the height of the block. The 
depth however will have little effect on the effectiveness of the cur- 
rent, if the block be of such a form and be so situated that the 
water can have access to nearly the whole of the lower surface. If 
therefore we take those blocks which are under the most favourable 
conditions for being moved (as we have a right to do), it is probable 
that it will be approximately correct to omit the effect of the depth 
of the water. In that case, supposing the form of the block as nearly 
