springing the paddles, and making them analogous to a fin. Paddle wheels act on a vessel in a mode analogous to the wheels of a road carriage in communicating the blows they receive to the vessel or body. The blows on the uneven road are interrupted by the springs; and if springs were applied to the paddle-wheels, they would interrupt the blows of the water. I was once arguing this matter with an engineer, and of great name too, and he maintained that the water was a spring. I closed the conversation with an argumentum ad hominem. “Try a practical conclusion, by trying its elasticity in a jump from Westminster Bridge.” He simply confounded mobility with elasticity, and considered a bag of small shot equivalent to a woolsack.
A simple mode of springing a paddle-wheel would be to permit it to revolve on its shafts, say three-fourths of its diameter, and limit the movement by elastic drag-links. The action would then be that of striking the water with an excess of force, the wheel would recoil round the shaft as the hand recoils on catching a cricket-ball, but with the advantage that the force of the recoil would exist on the paddle-wheel springs, and that force would be given out again with the reaction.
So with the screw. If the power were applied through the agency of springs the vibration would be diminished.
The paddle is analogous to the side fin of a fish—but the screw is analogous to the tail. If we compare the proportion of a screw to a vessel with the proportion of the tail to a whale’s body, we shall find the screw to be very small, and this is the true reason why we are obliged to use an enormous speed, shaking the vessel to pieces.
A screw working on a metal nut, whether worked fast or slow, makes the same progress with every revolution, that is, it has no slip. But when the screw has water for a nut, the slip is very great, and unless the speed of the screw is so great as to outstrip the mobility of the water, little propulsion of the vessel will take place, and slips will exist in proportion to the yield of the water, or the slowness of the vibration of the screw. If the size of the screw be increased, the speed may be reduced and the vibration may be reduced also. But the size of the shaft must be increased with the increase of the diameter of the screw, and the probability is that insufficient size, and excessive speed to make up for it is the great source of screw defects, originating in want of sufficient strength in the vessel to carry the required weight. Moreover, in sailing vessels it is needful to haul the screw out of the water when the fires are put out, and that renders heavy screws objectionable.
In war-vessels paddles are a serious objection on account of their exposure to shot. A steamer “winged” in action would be in an awkward predicament. But neither is the screw safe. Skilful gunners would aim just at the stern post, and render the screw unserviceable, putting the vessel in a worse condition than a “winged” steamer, which might shuffle off with the undamaged wing. How, then, is this difficulty to be met?
Simply by putting the propelling medium wholly under water. What do oars and paddles, and screws, all resolve themselves into. Simply a pumping action, neither more nor less. The oar is a lever working as a pump-handle, the blade of the oar is analogous to the pump-bucket. But it is not required to move the water, but only to use it as a fulcrum. If a common pump were laid horizontally on the surface of a pond, so as to float, with the handle upwards, and the handle worked in that position, there would be the same force, drawing the pump to the water as the water to the pump, the whole force being equal to the weight of the water in the bucket. In the vertical pump, the lifting of the bucket tends to create a vacuum, which the pressure of the atmosphere prevents; but in the horizontal pump the water follows the bucket with less resistance, and therefore a more rapid movement is required to move the pump to it, just as a rapid movement is required with the screw-propeller, which is a horizontal pump, and would lift water if enclosed in a case. If moved slowly the screw-propeller would scarcely move the vessel, but chiefly the water.
Many years have passed since Benjamin Franklin verified this fact seated on a ship’s pump floating in a pond at Boston, and, since then, vessels have from time to time been built for the especial purpose of pumping water through them as a means of propulsion.
Some very remarkable results were, by Mr. D. K. Clark, obtained from a vessel built for a deep sea fishing company in Scotland, the object of which was to use steam for rapid transit from the fishing-banks, without disturbing the fish, by the noise of paddles, or risking the destruction of nets by the screw. The arrangement was a circular pump, with a continuous revolution, drawing water through the stem of the vessel and pressing it aft over the quarter on each side, through revolving nozzles, which could be applied to steer the vessel, and with one nozzle turned forward, and the other aft, she would turn as on a pivot. It was practically making the water into a rope.
Whether revolving-pumps, i. e., propelling-screws, in cases, or a number of reciprocating pumps are best, is a matter for experiment; but, whether the one or the other, it is clearly desirable in large vessels to use two or more, instead of one, in order to render the machinery more manageable, and not to be without reserve in case of breakage.
The side-fins of fishes are apparently used chiefly for balancing, the great propelling power is on the tail. The tail is a reciprocating sculler or stern-oar, used with impulses as a skater moves over ice, but the whole fish is such a series of elasticity that little vibration or blow takes place, though the amount of power may be judged of by the force with which the tail of a whale strikes the water under the influence of rage or fear.
We can imagine a vessel covered all over with scales, like a Greek testudo, moving on hinges towards the stem and folding aft, forming a smooth surface when at rest, with hollows behind filled with water, each scale having a tight-fitting piston passing to the interior of the vessel through a stuffing-box. If all these scales were made to move simultaneously, so as to force out the water behind them, the result would be a spasmodic,