Gravity is the attraction between the earth and the bodies on the earth, which makes what we call weight of bodies. What do you understand by specific weight or gravity? It means the weight of a body as compared with the weight of an equal bulk of some other body taken as a standard--commonly water. Why do we say that certain metals--as, for example, platina or gold--are heavier than others, say, lead or iron? Because the former have a greater specific gravity. But is not a pound of gold as heavy as a pound of lead? Yes; but a lump of gold will be heavier than a lump of lead of equal bulk. Can we explain by this what we call floating? A body will float in water if its gravity is less than that of water; for example, wood floats for this reason in water, and a balloon in the air. Why does a portion of the floating body sink below the surface of the water? Because the body in order to float must displace a portion of water equal in weight to the whole floating body. But why do iron steamers float--iron being heavier than water? Because the steamer is not a solid piece of iron, but is hollow, and so increased in bulk; for that reason the weight of the vessel and its contents is less than that of an equal bulk of water. How can you ascertain that air has weight? We can do it by the barometer and by very many experiences in daily life. If one end of a straw be dipped into a vessel of water and the other end be sucked, the liquid will rise to the mouth. There we see the pressure of the outside air forces the liquid through the straw where the air was removed by sucking. Can you show the same by another instrument? Yes; the common water pump demonstrates the same as the straw. A tube is placed into the water, the air is sucked out from the tube by the movement of the pump, and the outside air presses the water through the tube. What are Artesian wells? Wells so named because they were made first at Artois, in France. They work on the principle that every liquid seeks its level. Of the rain which falls, a part soaks into the soil of mountains, until, coming to a layer of rocks or clay through which it cannot pass, it will collect and be stored up. If a hole be bored into this reservoir the water will rise in it. Do you know some other properties of air? It is the most necessary substance for our life; it is the vehicle of all odors and smells; it is the medium of all sounds, and brings to our ear and so to our mind an immense knowledge of the outside world; it is the cause of the beauty of the blue firmament or sky, of the aurora and twilight; it is the great nurse of the whole vegetable kingdom by clouds, rain, and dew. What is an Aneroid Barometer?