sence is required. We should
want in fact for great sources of energy some contrivance which shall
fulfil the same purpose as the accumulators do in an electrical
installation.

Even then, however, the financial consideration remains, as to whether
the cost of building the dam and maintaining the tide-mill in good
order will not on the whole exceed the original price and the charges
for the maintenance of a hundred horse power steam-engine. There
cannot be a doubt that in this epoch of the earth's history, so long
as the price of coal is only a few shillings a ton, the tide-mill,
even though we seem to get its power without current expense, is
vastly more expensive than a steam-engine. Indeed, Sir William Thomson
remarks, that wherever a suitable tidal basin could be found, it would
be nearly as easy to reclaim the land altogether from the sea. And if
this were in any locality where manufactures were possible, the
commercial value of forty acres of reclaimed land would greatly exceed
all the expenses attending the steam-engine. But when the time comes,
as come it apparently will, that the price of coal shall have risen to
several pounds a ton, the economical aspect of steam as compared with
other prime movers will be greatly altered; it will then no doubt be
found advantageous to utilize great sources of energy, such as Niagara
and the tides, which it is now more prudent to let run to waste.

For my argument, however, it matters little that the tides are not
constrained to do much useful work. They are always doing work of some
kind, whether that be merely heating the particles of water by
friction, or vaguely transporting sand from one part of the ocean to
the other. Useful work or useless work are alike for the purpose of my
argument. We know that work can never be done unless by the
consumption or transformation of energy. For each unit of work that is
done--whether by any machine or contrivance, by the muscles of man or
any other animal, by the winds, the waves, or the tides, or in any
other way whatever--a certain equivalent quantity of energy must have
been expended. When, therefore, we see any work being performed, we
may always look for the source of energy to which the machine owes its
efficiency. In fact, it is the old story illustrated, that perpetual
motion is impossible. A mechanical device, however ingenious may be
the construction, or however accurate the workmanship, can never
possess what is called pe