Science for the School and Family, Part I. Natural Philosophy

by Worthington Hooker

Fig. 126.

A swimmer, pressing the water downward and backward with his hands and feet, is carried along forward and upward by the reaction of the water. And in this case, as in every other, the greater the action the greater is the reaction; in other words, the more strongly he presses with his hands and feet, the more rapidly is he borne along by the reaction of the water against the pressure. A boat advances in proportion to the force with which the oars press against the water. So the rapidity of a steamboat depends on the force with which the paddles drive the water astern. Birds rise in the air by the reaction of the air against their wings as they are pressed downward. A sky-rocket pursues its rapid flight because a large quantity of gaseous matter issues from its lower end, and, being resisted by the reaction of the air, by its pressure throws the rocket upward. So if a ship fire guns from the stern its advance will be accelerated, but if from the bow it will be retarded. When a broadside is fired the ship inclines to the other side. In Fig. 126 (p. 136) is represented the plan of Barker's mill. It consists of a cylinder, c, arranged in a frame in such a way that it can revolve on the point upon which it rests. Water runs into it by a tube, p, and escapes by the branches, a and d. These are so arranged that the reaction upon the issuing water makes the cylinder revolve rapidly, causing the ends of the branches to whirl around as indicated by the dotted lines and arrows.

If a spring be compressed between two equal bodies, it will throw them off with equal velocities. If they are unequal, the velocity of the smaller body will be greater than that of the larger, and in proportion to its smallness. For this reason, when a ball issues from a cannon, though the cannon and the ball are equally acted upon by the elastic or expansive force of the gases set free by lighting the powder, the gun is moved but very little because the force is diffused through so large a mass, while the ball being so much smaller moves with great velocity. When a volcano throws stones from its crater the earth may be compared to the cannon, the stones to the ball, and the explosive materials throwing the stones to the exploding powder projecting the ball. As the cannon is moved as much as the ball, so is the earth moved as much as the stones, the only reason that it does not move as far and as rapidly as the stones being that the force is diffused through so large a bulk. These examples illustrate very well the relation of action and reaction, for whenever there is an action of one body upon another it is as if a spring were between the two bodies, acting equally upon both. When a man jumps from the ground it is as if a spring were compressed between him and the earth, and this expanding moves the earth exactly as much as it does the man. He really kicks the earth away from him. The motion of the earth is not obvious because it is diffused through so large a mass. The case is parallel to that of the ball and cannon. The same force is exerted upon the man and the earth, but the man, like the ball, moves the most, and in proportion to his comparative smallness. So when a bird hops from the ground, the earth moves as really as the bird. If the bird hop from a twig, you perceive that the twig is moved by the pressing down of the bird as it rises. When it starts from the ground it exerts the same downward pressure, and moves the earth as really as in the other case it did the twig.

183. Inertia Shown in the Communication of Motion.—What is meant by the inertia of matter you have already learned in § 48. This property is exemplified in the communication of motion to any body, or, in other words, in setting it in motion. Of this I will give some illustrations. When the sails of a vessel are first spread to the wind the vessel does not move swiftly at once, for some time is required for the force applied to overcome the inertia of so large a mass, and to put it in rapid motion. Horses make a greater effort to start a load than they do to keep it in motion after it is started. If one be standing up in a carriage, and the horses start off suddenly, he falls backward, because his body, from its inertia, does not readily and at once partake of the motion of the carriage. If a person start forward quickly with a waiter filled with glasses in his hands, the glasses will slide backward. So if a person start quickly from his chair with a cup of tea in his hand, the tea will be thrown backward upon him.

You see from the foregoing illustrations that it requires some time to communicate motion to any body. I will give some illustrations of this fact of a more striking character. If a ball be thrown against an open door it will move the whole door, and perhaps shut it; but the same ball if fired will pass through the door without moving it perceptibly. In the latter case its velocity is so great that there is not time enough to communicate motion to the whole door, and it moves only that part of it with which it comes in contact. A bullet thrown with but little force against a window will crack a whole pane of glass; but if shot from a pistol it merely makes a round hole. So, also, a cannon-ball having a great velocity may pass through the side of a ship, doing perhaps comparatively little damage, while one moving with much less velocity may do vastly more damage by splintering the wood to a considerable extent. For the same reason a rapid ball hitting a person may occasion less suffering and do less harm than a slow ball; for a rapid ball kills merely the parts which it touches, leaving the flesh around in a sound state, while the slow ball bruises over a large space. If a large pitcher filled with some heavy liquid be quickly taken up the handle will break, leaving the pitcher behind. Large dishes are often broken in this way when heavily loaded.

184. Inertia Shown in the Disposition of Motion to Continue.—Of this I will cite some illustrations. As in the case of the ship, in the first illustration in § 183, it takes time to communicate motion to the whole ship, or, in other words, to overcome its inertia, so when the ship is once in rapid motion it does not stop suddenly when the sails are taken down, but its inertia tending to keep it moving is gradually overcome by the resistance of the water. If one be standing up in a carriage in motion, and the horses suddenly stop, he will be thrown forward, for his body has a motion in common with the carriage, and from inertia is disposed to go on when the carriage stops. When you strike your foot against any thing to get the snow off, you give the foot and the snow a common motion together, then arresting the motion of the foot, the snow from inertia passes on. The same thing is illustrated in striking a book against any thing to get the dust off. If a ship strike upon a rock every thing on board which is loose is dashed forward. The earth as it revolves on its axis has a velocity at the equator of about 1000 miles an hour. If this revolution should be suddenly arrested every thing loose on its surface, having acquired the motion of the earth, would be at once thrown eastward, just as the furniture, etc., on board ship are dashed forward when it is stopped by running against a rock. All the houses, and monuments, and structures of every kind would fall prostrate eastward. All the cities on our Atlantic coast would be plunged into the ocean; and while the waters would leave the western shores of the Atlantic, they would overflow its eastern shores, and deluge the continent of Europe, as water in a vessel on board a ship that had struck an obstacle would be thrown forward over its side.

Fig. 127.

185. An Equestrian Feat.—In the feat represented in Fig. 127 the only exertion made by the rider is to raise himself sufficiently to pass over the cord, and he comes down again upon the horse's back, simply because of the motion which he has in common with the horse, his feet going in the path represented by the dotted line. If he should attempt to throw himself forward, as in leaping from the ground, he would go too far, and perhaps strike upon the horse's neck instead of his back. Skill in jumping from a moving carriage consists in making proper allowance for the forward motion which is had in common with the carriage. Most persons are apt to overdo the matter, and so come to the ground prostrate, and with more violence than is necessary.

186. A Case in Court.—A dashing young man driving a light phaeton ran against a heavy carriage. His father was induced by his son's representations to prosecute the driver of the carriage for driving too fast. A knowledge of motal inertia very readily decided the case. The son and his servant both declared that the shock of the carriage was so great against the phaeton that they were thrown over the horses' heads. They thus proved themselves guilty of the fast driving, for it was their own rapid motion that threw them out when the phaeton was stopped by running against the carriage. The following case is a parallel one. If two boats, the one of large size sailing slowly up stream, the other a small one sailing rapidly down, run against each other, a man standing in the bow of the one going down will be thrown much farther forward than one standing in the bow of the other.