If you take a blown-up bladder under water and let go of it, it will fly up to the surface of the water and will swim on it. Just so, when water is boiled in a pot, it becomes light at the bottom, over the fire,—it is turned into a gas; and when a little of that water-gas is collected it goes up as a bubble. First comes up one bubble, then another, and when the whole water is heated, the bubbles come up without stopping. Then the water boils. Just as the bubbles leap to the surface, full of vapory water, because they are lighter than water, just so will a bladder which is filled with hydrogen, or with hot air, rise, because hot air is lighter than cold air, and hydrogen is lighter than any other gases. Balloons are made with hydrogen or with hot air. With hydrogen they are made as follows: They make a large bladder, attach it by ropes to posts, and fill it with hydrogen. The moment the ropes are untied, the balloon flies up in the air, and keeps flying up unt...