Pumps are mostly used by people in the country, for the purpose of obtaining water from the wells which have been sunk in the earth. When does smoke ascend through the air? When smoke is lighter than the air, it ascends through it. When does smoke descend through the air? When smoke is heavier than the air, it descends through it. Why does a cork rise to the surface of the water? Because the cork is lighter than the same bulk of water; hence, it ascends to the surface. Why does heated air ascend? Because it is lighter than the cold air. If a feather be put in the heated air, will it be carried up with the air? The feather will be carried up with the air. If warm air be confined in a bag, will it rise up and carry the bag with it? It will rise up and carry the bag with it. What might this bag be called? It might be called a _balloon_. What is a balloon? It is a bag filled with gas, and used for sailing in the air. [Illustration: Fig. 19.] What is placed over this bag? A network of ropes is placed over it, and they are so arranged as to extend beneath the bag, as is represented in Fig. 19. What is fastened to these ropes? A car or basket is fastened to these ropes. What is the car or basket for? It is to carry up those who wish to ascend with the balloon. How large are balloons usually made? They are from twenty to thirty feet in diameter. With what are balloons filled? Balloons are filled with a gas lighter than air. What gas is often used for filling balloons? The coal gas, such as is used to light our cities at night. What lighter kind of gas is sometimes used? Hydrogen gas is sometimes used. Is hydrogen gas much lighter than the air? It is so much lighter than the air, that fifteen cubic feet of hydrogen gas weigh no more than one cubic foot of air. How much less will a balloon thirty feet in diameter weigh, when filled with hydrogen gas, than when filled with air? It will weigh about ten thousand pounds less. How much less, when filled with coal gas than when filled with air? About two thousand pounds less. How much force then is necessary to hold the balloon filled with coal gas to the earth? A force of two thousand pounds, because the balloon is two thousand pounds lighter than the same bulk of air. If a weight of only one thousand pounds were fastened to the balloon, what would result? The balloon would rise in the air and carry the weight of one thousand pounds with it. How high will the balloon ascend? It will ascend until the air displaced is of the same weight as the balloon. What will then become of the balloon? It will be carried about in the currents of air. With what is the car attached to the balloon loaded? It is usually loaded with bags of sand, which serve as a weight to keep it from rising too high. When those in the car wish to go higher, what do they do? They throw the sand out, so as to lighten the car; then it will rise higher.