It may be urged, and with propriety, that the results obtained in practice with the jet propeller are more favorable than our reasoning would indicate as possible; but it will be seen that we have taken no notice of conditions which seriously affect the performance of a screw. There is no doubt that it puts water in motion not astern. It twists it up in a rope, so to speak. Its skin frictional resistance is very great. In a word, in comparing the hydraulic system with the normal system, we are comparing two very imperfect things together; but the fact remains, and applies up to a certain point, that the hydraulic propeller must be very inefficient, because it, of all propellers, drives the smallest quantity of water astern at the highest velocity. There is, moreover, another and a very serious defect in the hydraulic propeller as usually made, which is that every ton of water passed through it has the velocity of the ship herself suddenly imparted to it. That is to say, the ship has to drag water with her. To illustrate our meaning, let us suppose that a canal boat passes below a stage or platform a mile long, on which are arranged a series of sacks of corn. Let it further be supposed that as the canal boat passes along the platform, at a speed of say five miles an hour, one sack shall be dropped into the boat and another dropped overboard continuously. It is evident that each sack, while it remains in the boat, will have a speed the same as that of the boat, though it had none before. Work consequently is done on each sack, in overcoming its inertia by imparting a velocity of five miles an hour to it, and all this work must be done by the horse towing on the bank. In like manner the hydraulic propeller boat is continually taking in tons of water, imparting her own velocity to them, and then throwing them overboard. The loss of efficiency from this source may become enormous. So great, indeed, is the resistance due to this cause that it precludes the notion of anything like high speeds being attained. We do not mean to assert that a moderate degree of efficiency may not be got from hydraulic propulsion, but it can only be had by making the quantity of water sent astern as great as possible and its velocity as small as possible. That is to say, very large nozzles must be employed. Again, provision will have to be made for sending the water through the propeller in such a way that it shall have as little as possible of the motion of the ship imparted to it. But as soon as we begin to reduce these principles to practice, it will be seen that we get something very like a paddle wheel hung in the middle of the boat and working through an aperture in her hull, or else a screw propeller put into a tube traversing her from stem to stern.