n
some of our great rivers they are of the utmost consequence. These
currents of water can, like water-streams of any other kind, be made to
do useful work. We can, for instance, impound the rising water in a
reservoir, and as the tide falls we can compel the enclosed water to
work a water-wheel before it returns to the sea. We have, indeed, here a
source of actual power; but it is only in very unusual circumstances
that it is found to be economical to use the tides for this purpose. The
question can be submitted to calculation, and the area of the reservoir
can be computed which would retain sufficient water to work a
water-wheel of given horse-power. It can be shown that the area of the
reservoir necessary to impound water enough to produce 100 horse-power
would be 40 acres. The whole question is then reduced to the simple one
of expense: would the construction and the maintenance of this reservoir
be more or less costly than the erection and the maintenance of a
steam-engine of equivalent power? In most cases it would seem that the
latter would be by far the cheaper; at all events, we do not practically
find tidal engines in use, so that the power of the tides is now running
to waste. The economical aspects of the case may, however, be very
profoundly altered at some remote epoch, when our stores of fuel, now
so lavishly expended, give appreciable signs of approaching exhaustion.

The tides are, however, _doing work_ of one kind or another. A tide in a
river estuary will sometimes scour away a bank and carry its materials
elsewhere. We have here work done and energy consumed, just as much as
if the same task had been accomplished by engineers directing the
powerful arms of navvies. We know that work cannot be done without the
consumption of energy in some of its forms; whence, then, comes the
energy which supplies the power of the tides? At a first glance the
answer to this question seems a very obvious one. Have we not said that
the tides are caused by the moon? and must not the energy, therefore, be
derived from the moon? This seems plain enough, but, unfortunately, it
is not true. It is one of those cases by no means infrequent in
Dynamics, where the truth is widely different from that which seems to
be the case. An illustration will perhaps make the matter clearer. When
a rifle is fired, it is the finger of the rifleman that pulls the
trigger; but are we, then, to say that the energy by which the bullet
has been