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.