Suppose that you have a tall receiver, Fig. 128, on the air-pump, and a piece of lead and a feather are placed at its upper part in such a way that they can be made to fall at the same instant. Exhaust the air, and then let them fall. They will go down side by side, as represented by the figure, and reach the bottom of the receiver at the same time, because there is no air there to resist the progress of the feather. The toy called the water-hammer illustrates the same thing. When water falls through the air the resistance of the air tends to separate its particles, as we see in the falling of water thrown up by a fountain. In the water-hammer, which is a closed tube containing a little water and no air, when the water is made to fall from one end to the other, as there is no air to divide it, it falls as one mass, and gives a sharp sound like the blow of a hammer. An instrument essentially like this can be made with a thin glass flask. Put a little water in it, and, after heating it to boiling over a spirit-lamp, cork the flask tightly, and then leave the water to cool. As all the space above the water was filled with steam when the flask was corked, it is a vacuum now that the steam is condensed. [Illustration: Fig. 129.] 193. =Relation of Bulk to the Resistance of Liquids and Gases.=--You have already seen, in § 192, that the more surface a body has in proportion to its weight the greater is the resistance of the air to its motion. This truth, which applies to liquids as well as to airs or gaseous substances, explains the fact that small bodies meet with proportionately more resistance than large ones. The body B, Fig. 129, you see is made up of eight cubes of the size of the cube _a_, that is, it has eight times the quantity of matter that _a_ has. Now if B were moving through air or water, any of its sides pushing the water before it would meet with only four times the resistance that a side of _a_ would, for its surface is only four times as large, and yet the body is eight times as large as _a_. And the greater the difference of size the greater is the difference of resistance. If B were a cube twenty-seven times as large as _a_ it would meet with only nine times as much resistance. You see here the reason that shells and cannon-balls can be thrown much farther than bullets and small shot. The sportsman does not throw away his shot by foolishly aiming at birds at great distances, and yet shells and large cannon-balls can be thrown the distance of several miles. The difference is not in the degree of velocity which the powder produces, but in the resistance of the air. It is for the same reason that rain falls with greater rapidity than drizzling mist.