The going up of a rocket compared to jumping and flying.
Powder is used in various ways. Some kinds of fire-works are made in such a way that the powder does not burn all at once, as it does in a gun or cannon. You know that when a rocket goes up, it is not sent up by one blast of the powder, as a ball is sent out of a gun. The powder is placed in the tail of the rocket, which is so made that the powder burns all the time that it is going up, the last of it making an explosion high up in the air, scattering the sparks which fall in so beautiful a shower. Now, did you ever think just how it is that the rocket is made to go up so swiftly? It is the gas of the burning powder which streams out from its tail all the time that makes it go up. This pushes down against the air, and it is the resistance of the air to this that raises the rocket. It is just as the resistance of the air to the downward stroke of the bird’s wings raises the bird. It is also just as, in jumping up off the ground, the resistance of the ground to your feet makes you go up. You press with your feet on the ground, and so the rocket presses with its gas on the air; and so long as gas keeps coming out of its tail to press on the air, the rocket keeps going up. When the gas is exhausted the rocket comes down.
You have sometimes seen whirling wheels in fire-works. The powder in the wheel is arranged as you see here; and as it burns, the resistance of the air to the gas makes the wheel fly around, for the same reason that it makes the rocket go up into the air.
Questions.—What is said about powder when no fire touches it? How is the power of burning powder shown? How are its effects produced? What is the smoke from powder? What is said about the quickness with which powder changes into gas? Tell about the boy that split a log of wood with powder. Give the comparison about steam. How is it that the gas made by burning powder makes a ball go out of a cannon or gun so swiftly? Give the comparison of the quill and the ball of paper. Why does the gas from a cannon spread after it gets out? Tell what is said about blasting rocks. Explain how a rocket is made to go up in the air. What is the comparison about flying and about jumping? What is said about the whirling wheel in fire-works?
CHAPTER X.
POP-GUNS.
Operation of the pop-gun explained.
Every boy and girl has played with a pop-gun, but did you ever think how it works? I will tell you about this.
You know that the cork does not fly out till the rod is pushed a considerable way down into the tube or barrel of the gun, and then it flies out all at once with a popping noise. What makes it fly out? It is not the rod alone, for it does not touch the cork. It is the air that is between the rod and the cork that gives it the push that makes it fly out, and it gives so quick a push as to make the pop.