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, then we can readily understand that the water composing such a sea would offer no resistance to being pushed astern by paddle or screw.  When a gun is fired, the weapon moves in one direction—­this is called its recoil—­while the shot moves in another direction.  The same principal—­pace Professor Greenhill—­operates to cause the movement of a ship.  The water is driven in one direction, the ship in another.  Now, Professor Rankine has laid down the proposition that, other things being equal, that propeller must be most efficient which sends the largest quantity of water astern at the slowest speed.  This is a very important proposition, and it should be fully grasped and understood in all