rgely neglected for two or three generations, are now commanding
attention as valuable and highly profitable sources of power. This is
only to be regarded as forming the small beginning of a movement
which, in the coming century, will "acquire strength by going," and
which most probably will, in less than a hundred years, have produced
changes in the industrial world comparable to those brought about by
the invention of the steam-engine.

Lord Kelvin, in the year 1881, briefly, but very significantly,
classified the sources of power available to man under the five
primary headings of tides, food, fuel, wind, and rain. Food is the
generator of animal energy, fuel that of the power obtained from steam
and other mechanical expansive engines; rain, as it falls on the
hill-tops and descends in long lines of natural force to the sea
coasts, furnishes power to the water-wheel; while wind may be utilised
to generate mechanical energy through the agency of windmills and
other contrivances. The tides as a source of useful power have hardly
yet begun to make their influence felt, and indeed the possibility of
largely using them is still a matter of doubt. The relative advantages
of reclaiming a given area of soil for purposes of cultivation, and of
converting the same land into a tidal basin in order to generate power
through the inward and outward flow of the sea-water, were contrasted
by Lord Kelvin in the statement of a problem as follows: Which is the
more valuable--an agricultural area of forty acres or an available
source of energy equal to one hundred horse-power? The data for the
solution of such a question are obviously not at hand, unless the
quality of the land, its relative nearness to the position at which
power might be required, and several other factors in its economic
application have been supplied. Still, the fact remains that very
large quantities of the coastal land and a considerable quantity of
expensive work would be needed for the generation, by means of the
tides, of any really material quantity of power.

It is strange that, while so much has been written and spoken about
the possibility of turning the energy of the tides to account for
power in the service of man, comparatively little attention has been
paid to the problem of similarly utilising the wave-power, which goes
to waste in such inconceivably huge quantities. Where the tidal force
elevates and depresses the sea-water on a shore, through a vert