In Search of a Son

by William S. Walsh

"The machine," he said, "is called an air-pump because it is intended to exhaust air contained in a vase or other receptacle. To exhaust the air in a vase is to make a vacuum in that vase. You will see that this machine is composed of two cylinders, or pump-barrels, out of which there comes a tube, which opens in the centre of this disk of glass. Upon this disk we carefully place this globe of glass; and now we are going to exhaust the air contained in the globe."

"We are going to make a vacuum," said Miette.

"Exactly." And Monsieur Roger commenced to work the lever. "You will take notice," he said, "that when the lever is lowered at the left the round piece of leather placed in the cylinder on the left side is lowered, and that the bit of leather in the right-hand cylinder is raised. In the same way, when the lever is lowered at the right, it is the right-hand piece of leather which is lowered, while the piece of leather at the left is raised in its turn. These round bits of leather, whose importance is considerable, are called pistons. Each piston is hollow and opens into the air on top, while at the bottom, which communicates with that portion of the cylinder situated below the piston, there is a little hole, which is stopped by a valve. This valve is composed of a little round bit of metal, bearing on top a vertical stem, around which is rolled a spring somewhat in the shape of a coil or ringlet. The ends of this spring rest on one side on a little bit of metal, on the other on a fixed rest, pierced by a hole in which the stem of the valve can freely go up and down. When I work the lever, as I am doing now, you see that on the left side the piston lowers itself in the cylinder, and that the piston on the right is raised. Now, what is going on in the interior of each cylinder? The piston of the left, in lowering, disturbs the air contained in the cylinder,—it forces it down, it compresses it. Under this compression the coiled spring gives way, the round bit of metal is raised, and opens the little hole which puts the under part of the piston in communication with the atmosphere. The air contained in the cylinder passes in this way across the piston and disperses itself in the air which surrounds us. But the spring makes the bit of metal fall back again and closes the communication in the right-hand cylinder as soon as the piston commences to rise and the pressure of the air in the cylinder is not greater than the pressure of the atmosphere outside. Lastly, the tube which unites the cylinders to the glass globe opens at the end of each cylinder, but a little on the side. It is closed by a little cork, carried by a metal stem which traverses the whole piston. When I cause one of the pistons to lower, the piston brings the stem down with it. The cork at once comes in contact with the hole, which closes; the stem is then stopped, but the piston continues to descend by sliding over it. In the other cylinder, in which the piston is raised, it commences by raising the stem, which re-establishes communication with the glass globe; but as soon as the top of the stem comes in contact with the upper part of the cylinder, it stops and the piston glides over it and continues to rise."

"In this fashion the movements of the cork are very small, and it opens and shuts the orifice as soon as one of the pistons begins to descend and the other begins to ascend. Consequently, by working the lever for a certain space of time, I will finish by exhausting the globe of all the air which it contains."

"May I try to exhaust it?" asked Miette, timidly.

"Try your hand, Miss Miette," answered Monsieur Roger.

Miette began to work the lever of the air-pump, which she did at first very easily, but soon she stopped.

"I cannot do it any more," said she.

"Why?"