ficult to make those who believe in such devices understand
that there are valid reasons standing in the way of their adoption.
One such device is hydraulic propulsion. A correspondent in a recent
impression suggested its immediate and extended use in yachts at all
events, and we willingly published his letter, because the system does
no doubt lend itself very freely to adoption for a particular class of
yachts, namely, those provided with auxiliary power only. But because
this is the case it must not be assumed that the jet propeller is
better than screw or paddle-wheel propulsion; and it is just as well,
before, correspondence extends further, that we should explain why and
in what way it is not satisfactory. The arguments to be urged in favor
of hydraulic propulsion are many and cogent; but it will not fail to
strike our readers, we think, that all these arguments refer, not to
the efficiency of the system, but to its convenience. A ship with a
hydraulic propeller can sail without let or hindrance; a powerful pump
is provided, which will deal with an enormous leak, and so on. If all
the good things which hydraulic propulsion promises could be had
combined with a fair efficiency, then the days of the screw propeller
and the paddle wheel would be numbered; but the efficiency of the
hydraulic propeller is very low, and we hope to make the reason why it
is low intelligible to readers who are ignorant of mathematics. Those
who are not ignorant of them will find no difficulty in applying them
to what we have to say, and arriving at similar conclusions in a
different way.

Professor Greenhill has advanced in our pages a new theory of the
screw propeller. As the series of papers in which he puts forward his
theory is not complete, we shall not in any way criticise it; but we
must point out that the view he takes is not that taken by other
writers and reasoners on the subject, and in any case it will not
apply to hydraulic propulsion. For these reasons we shall adhere in
what we are about to advance to the propositions laid down by
Professor Rankine, as the exponent of the hitherto received theory of
the whole subject. When a screw or paddle wheel is put in motion, a
body of water is driven astern and the ship is driven ahead. Water,
from its excessive mobility, is incapable of giving any resistance to
the screw or paddle save that due to its inertia. If, for example, we
conceive of the existence of a sea without any inertia,