The Action of a Ship's Helm. 225 tiller is above the deck, and the steersman applies his hand directly to it; but in large ships it is below, and is moved by ropes or chains leading to the axle of the wheel on the deck, where the steersman stands with the compass before him. While the rudder points directly astern, as shown by the line a, it does not affect the vessel's course; but if it be inclined ever so little to one side, as is the line, b, on the left or larboard side, the water offers greater resistance in the direction, cb, and the stern. moves to the right or starboard side-an action equivalent to pulling the bow to the left or larboard. a Fig. 98. 378. A ship or boat might be made to steer itself, by placing a powerful vane on the mast-head, and connecting that with the tiller-ropes by two arms projecting from its axis. To make the ship sail directly before the wind, the tiller-ropes would have to be connected with the arms of the vane so that the helm should be in the middle position when the vane was pointing directly forward. Should the vessel then by any cause deviate from her course, the vane by its changed position with respect to her keel, would produce a corresponding change on the position of the helm, just such as to bring her back to her course. By adjusting such a vane and rudder to each other in different ways, any other desired course might be obtained, which would alter only with the wind. The vane, to have the necessary power, would require to be of large size --a wide hoop, for instance, with canvas stretched upon it; and the rudder, to turn with little force, might be hung on an axis passed nearly through its middle, instead of, as usual, by hinges at one edge. So long as the wind kept the same direction, the course of the vessel might in this way be exactly prescribed beforehand. 379. As fluids act on surfaces in a direction perpendicular to them, the water on the right side of a ship's bow is always pressing it towards the left side; but owing to the equivalent and contrary pressure on the left side, the ship holds her course evenly between the two, or straight-forwards. When a ship, however, owing to a side wind, lies over or heels, as it is called, that side of the bow which sinks most is more pressed than the other; and were there not then made a counteracting inclination of the rudder, constituting what is called weather-helm, the ship's head would come round to the wind. Now ships so rarely have the wind exactly astern and the masts quite erect, that to diminish the almost constant necessity for weather-helm, the mast or masts, and conse 226 Oblique Action: Rifling: Windmills. quently the mass of the sails, are placed nearer to the bow than to the stern. Again, because the bow of a ship is oblique below as well as on the sides, the water, when she moves, is constantly tending to lift the bow; hence when a vessel is dragged by a low horizontal rope, as a boat is when attached to a sailing ship's stern, or is moved by paddle-wheels, like steam-boats, the bow rises more or less out of the water, and the stern sinks in the hollow or furrow of the track; but when a ship is driven by sails, which are high on the mast, and are acting therefore as by a long lever to depress the bow, the two opposing tendencies just balance each other, and the vessel sails evenly along. In 380. It may be observed here that, while greater breadth of prow causes increased resistance to the advancing motion of a ship, greater length of hull has very little influence, for the prow opens the way for any length of hull, and there can arise only a little increase of friction from increase of length. The same principle explains why, in artillery practice, elongated shells or shot can be thrown much farther than globular masses of the same weight. the small-bore rifles of the present day, as has been experimentally proved by Sir Joseph Whitworth, the length should be at least three times the diameter; and a like rule holds for ordnance projectiles. A 9-pounder Whitworth gun has been found to throw a projectile, four diameters long, to a distance of fully six miles. Such a range would to our ancestors have appeared as incredible as the labours of Hercules. 381. The common windmill furnishes another important illustration of oblique fluid action. The face of the windmill, as a whole, is turned directly to the wind, but the faces of the four flat vanes or sails, which ap pear as the arms of the great wheel, are indi. vidually oblique. Thus the edge, a, of the vane, a e (fig. 99), is more forward as regards the coming wind or a spectator in front, than the edge, e; and the action of the wind, therefore, being perpendicular to the oblique surface, a c, pushes it in a degree towards a, as the point of the arrow shows. The same remark applies to each of the other vanes, where the edges, b, c, and d, are in front, and those marked by the fainter lines are farther back; so that each vane produces an equal effect in turning Fig. 99. Examples of Oblique Fluid-action. 227 the wheel. By the "resolution of forces" (Arts. 129 and 130), we can tell in what proportions the force of the wind is exerted to push the wheel backwards against its supports, and to turn it round. Windmills were first used in Europe in the fourteenth century, and they are still of great importance in countries where there are no waterfalls, and where fuel for steam-engines is expensive. In some of the richest Continental landscapes every height is crowned by its busy windmill, grinding corn, or sawing wood, or pressing oil-seeds; and over the plains, similar wheels are pumping water for domestic use, or incessantly draining the land. The smoke-jack of our chimneys is a small windmill, driven by the ascending current of air in the chimney. The feathering of an arrow acts in part on the principle of the windmill. The feathery projection from the shaft is not quite straight, but winds round it a little, like the thread of a screw; and the arrow, therefore, constantly turns as it flies, and goes straight to its object even if the shaft itself be somewhat bent, because any deviation is constantly correcting itself. The rifling in fire-arms consists of spiral furrows or threads along the interior surface of the barrel, so that the bullet in passing out receives a turning motion round the line of its flight, corresponding to that of a feathered arrow, and produces similar results. A oullet which receives any other turning motion than round the line of its course and most bullets from an unrifled barrel do acquire such, owing to some irregularity of their form, or to unequal friction at the mouth of the piece—is sure to deviate from its course, because unequally pressed or resisted by the atmosphere. The greater friction and pressure from which it turns away, is on that side of the ball which is advancing more quickly than the centre. A good rifle fixed to its place will send a succession of shots through the hole made in the target by the first shot. 382. It was supposed by some that a wheel which the wind turned by direct action on flat projections round the circumference, as water turns common water-wheels, would be more effective than the windmill-wheel above described, which is turned by oblique pressure on its face, and accordingly a wheel like a water-wheel, only with broader vanes, was constructed and placed so that only one side was exposed to the wind-but it was found to be a comparatively powerless machine. The wider expanse of the oblique-vaned face was found to be much more than a compensa. tion for the obliquity of the wind's action upon it. 228 Examples of Oblique Fluid-reaction. 383. A windmill-wheel, made to turn during a calm by force applied to its axle, is, according to the law of action and reaction being equal and contrary, pressed endways with nearly the force used in turning it, owing to the reaction of the still air through which its oblique vanes are caused to sweep. If, in such an experiment, the windmill-wheel is supported on the mast of a floating boat, it urges the boat along with the force referred to. Such a wheel placed in a short cylindrical tube or passage has been used to produce an artificial wind or air-current for the ventilation of closed spaces. A small wheel of the kind, carried in the hand of a person walking along in a calm, turns as if wind were blowing on it at the rate of the walker's motion, and if connected with a train of wheels and an index, like those of the common gasmeter, it indicates the length of space passed through. Such a wheel placed in the wind tells the speed of the wind. And such a wheel fixed on the end of a spindle and caused to spin round like a humming-top, rises into the air, constituting a kind of flying machine. 384. There are situations where it would be advantageous to use water-wheels constructed with arms and oblique surfaces like the common windmill wheel: namely, in streams deep enough to allow the whole wheel to be immersed. Because water is more than 800 times heavier than air, bulk for bulk, its force, either acting when itself in motion, or in resisting and re-acting against other motions, is proportionally great. This explains the marvellous efficacy of such a water-wheel when used on board ship, as now, under the name of screw-propeller, constituting the great instrument of steamnavigation. 385. The so-called screw-propeller, when first offered to notice, was far from being completely understood either by those who proposed it—several of whom had taken patents for it as a novelty—or by those opposed to it as being less effective than the paddle-wheel. The advocates for it first used a screw of several turns of the flange, whence its name was derived; but they soon found that two turns like the common cork-screw answered better than three or more; then that one turn was better than two; and, lastly, that half a turn, divided into two opposite arms, like two arms of a windmill, answered best of all. At first, few on either side seemed to be fully aware of the following facts : 1. That this propeller differed from the paddle-wheel, almost exactly as the commcn broad-faced windmill with oblique surfaces The Screw-propeller. 229 differs from the common wheel partially exposed to the wind, as described in Arts. 381, 382. 2. That the so-called mechanical power-the screw, does not at all waste force on account of the obliquity of the surfaces of contact, provided the external screw or nut is firm or unyielding. 3. That a fluid surface if pressed upon by a solid which passes as rapidly along or over it as the propeller-surface passes over the water-surface against which it bears, resists nearly as effectually as a solid surface would, and that the propeller, therefore, when the pitch of the flange is properly adjusted, loses less force by the yielding of the water than a paddle-wheel does. This very important and little-considered fact, of the almost solid resistance of a fluid to a rapidly passing pressure, is seen in such cases as the following. A cannon ball always rebounds from the surface of water, almost as from the surface of a stone pavement, when it is shot in a nearly horizontal direction. The ball, when it descends and touches, is resisted by the inertia and reaction, not of its own bulk of water, but of perhaps a hundred times as much, within the one second or two of contact as it passes quickly along ; and it therefore rebounds and relapses several times before its motion is exhausted. The like happens when a boy at play throws a flat pebble or oyster shell along the surface of a pond and sees it skip and leap forward. The same principle is illustrated by the longresisted and slow descent of a broad leaf falling from a tree, when it zig-zags and thereby touches much air,—in the slow slanting motion of the boomerang descending,—in the mode of flight of the great albatross, whose wings appear scarcely to move as he glides about in the atmosphere supported by the resistance offered to the under surface of his expanded wings by the new air, which he every instant reaches. The author, when he published this work, before the screw-propeller had been tried at sea, explained the true theory by reference to the windmill-wheel, &c., as here repeated; but not having had occasion to consider the matter closely, he did not then question the opinions which had been given by eminent practical men, that there would be loss of power in substituting the oblique, lateral, or twisting pressure of the screw for the direct backward pressure of the paddle-wheel. After a time, however, learning by accident that a friend of his who knew little of science, had been induced to lend a large sum of money to build a vessel of size sufficient to test completely the qualities of the screw, he was led to review the subject in |