509. We shall illustrate this remarkable property by an experiment. The principle of doing so is as follows:—Suppose we take two bodies, a and b. If these be held at the same height, and released together, of course they reach the ground at the same instant; but if a, instead of being merely dropped, be projected with a horizontal velocity at the same moment that b is released, it is still found that a and b strike the floor simultaneously.
510. You may very simply try this without special apparatus. In your left hand hold a marble, and drop it at the same instant that your right hand throws another marble horizontally. It will be seen that the two marbles reach the ground together.
511. A more accurate mode of making the experiment is shown by the contrivance of [Fig. 68].
Fig. 68.
In this we have an arrangement by which we ensure that one ball shall be released just as the other is projected. At a b is shown a piece of wood about 2" thick; the circular portion (2' radius) on which the ball rests is grooved, so that the ball only touches the two edges and not the bottom of the groove. Each edge of the groove is covered with tinfoil c, but the pieces of tinfoil on the two sides must not communicate. One edge is connected with one pole of the battery k, and the other edge with the other pole, but the current is unable to pass until a communication by a conductor is opened between the two edges. The ball g supplies the bridge; it is covered with tinfoil, and therefore, as long as it rests upon the edges, the circuit is complete; the groove is so placed that the tangent to it at the lowest point b is horizontal, and therefore, when the ball rolls down the curve, it is projected from the bottom in a horizontal direction. An india-rubber spring is used to propel the ball; and by drawing it back when embraced by the spring, I can communicate to the missile a velocity which can be varied at pleasure. At h we have an electro-magnet, the wire around which forms part of the circuit we have been considering. This magnet is so placed that a ball suspended from it is precisely at the same height above the floor as the tinned ball is at the moment when it leaves the groove.
512. We now understand the mode of experimenting. So long as the tinned ball g remains on the curve the bridge is complete, the current passes, and the electro-magnet will sustain h, but the moment g leaves the curve, h is allowed to fall. We invariably find that whatever be the velocity with which g is projected, it reaches the ground at the same instant as h arrives there. Various dotted lines in the figure show the different paths which g may traverse; but whether it fall at d, at e, or at f, the time of descent is the same as that taken by h. Of course, if g were not projected horizontally, we should not have arrived at this result; all we assert is that whatever be the motion of a body, it will (when possible) be at the end of a second, sixteen feet nearer the earth than it would have been if gravity had not acted. If the body be projected horizontally, its descent is due to gravity alone, and is neither accelerated nor retarded by the horizontal velocity. What this experiment proves is, that the mere fact of a body having velocity does not affect the action of gravity thereon.
513. Though we have only shown that a horizontal velocity does not affect the action of gravity, yet neither does a velocity in any direction. This is verified, like the first law of motion, by the accordance between the consequences deduced from it and the facts of observation.
514. We may summarize these results by saying that no matter what be the material of which a particle is composed, whether it be heavy or light, moving or at rest, if no force but gravity act upon the particle for t seconds, it will then be 16t² feet nearer the earth than it would have been had gravity not acted.
