nd will be filled; then at the moment of high water close the
sluice, and the pond-full will be impounded. If at low tide the sluice
be opened the water will rush tumultuously out. Now suppose that a
water-wheel be provided, so that the rapid rush of water from the exit
shall fall upon its blades; then a source of power is obviously the
result.

At present, however, such a contrivance would naturally find no
advocates, for of course the commercial aspect of the question is that
which will decide whether the scheme is practicable and economical.
The issue indeed can be very simply stated. Suppose that a given
quantity of power be required--let us say that of one hundred horse.
Then we have to consider the conditions under which a contrivance of
the kind we have sketched shall yield a power of this amount. Sir
William Thomson, in a very interesting address to the British
Association at York in 1881, discussed this question, and I shall here
make use of the facts he brought forward on that occasion. He showed
that to obtain as much power as could be produced by a steam-engine
of one hundred horse power, a very large reservoir would be required.
It is doubtful indeed whether there would be many localities on the
earth which would be suitable for the purpose. Suppose, however, an
estuary could be found which had an area of forty acres; then if a
wall were thrown across the mouth so that the tide could be impounded,
the total amount of power that could be yielded by a water-wheel
worked by the incessant influx and efflux of the tide would be equal
to that yielded by the one hundred horse engine, running continuously
from one end of the year to the other.

There are many drawbacks to a tide-mill of this description. In the
first place, its situation would naturally be far removed from other
conveniences necessary for manufacturing purposes. Then too there is
the great irregularity in the way in which the power is rendered
available. At certain periods during the twenty-four hours the mill
would stop running, and the hours when this happened would be
constantly changing. The inconvenience from the manufacturer's point
of view of a deficiency of power during neap tides might not be
compensated by the fact that he had an excessive supply of power at
spring-tides. Before tide-mills could be suitable for manufacturing
purposes, some means must be found for storing away the energy when it
is redundant, and applying it when its pre