"We have had occasion to refer to the air-pump in some of our lessons," said Mr. Wilson. "This morning we will have a look at the pump itself, to find out how it is made and how it acts.
"It consists, as you may see, of two parts-a pump, and a large glass bell, commonly called the receiver. The pump is very similar in construction to the common suction- pump. It consists of a barrel, and an air-tight piston capable of moving up and down in it. It is made of brass, and every part is most carefully fitted.
"As in the suction-pump, the piston is furnished with a valve opening outwards, and there is also at the bottom of the barrel another valve, which opens in the same direction. You know that the whole purpose of a valve is to allow the easy and ready flow of a fluid in one direction, and toprevent it from flowing back again. The first attempt the fluid makes to flow back simply shuts the valve, and the passage is closed.
"Below the barrel is a brass pipe leading from the pump to the second part of the instrument-the receiver. This pipe is furnished with a stop-cock. When the cock isturned on, the way along the pipe is open, but when it is turned off, and so closed, not a particle of air can find its way through.
"The receiver is simply a large glass bell, which fits closely on a brass plate. In the brass plate is a hole into which fits-perfectly air-tight the pipe leading from the barrel of the pump.
"The whole object of the contrivance is to remove the air from the receiver, so as to leave an empty space, or vacuum.
"Let us turn the cocks on, and commence pumping with the piston close down on the suction- valve at the bottom of the barrel. That valve is quite closed, and so is the piston-valve above. Now, as we raise the piston, the pressure of the air in the barrel closes its valve, and all that air is forced out at the top.
"But what has been going on below the piston? The raising of the piston has made (or tried to make) a vacuum above the suction-valve.
"So far you have noticed, no doubt, that the construction and working of the air-pump and the common suction- pump are exactly the same.
"Now I want you to consider the nature of the two fluids. Water is a liquid, and rises in a body into the barrel, as soon as the air-pressure is removed. The air to be removed from the receiver of the air-pump is a gas. It has no cohesion between its particles. They spread out from each other, so as to fill the greatest possible space.
"Keeping this in mind, let us return now to the vacuum which the pumping makes in the barrel of the air-pump. As this is being done, the air in the receiver and the pipe leading from it expands, forces open the suction-valve, fills the barrel again, and prevents a vacuum. There is air still in the receiver, the pipe, and the barrel, but it is not so dense as it was before the pumping. Some of it has been removed, and the rest has been made to expand, so as to fill a greater space.
"Now we will force the piston down again. This downward movement of the piston compresses the air in the barrel and closes the suction-valve; while at the same time this compressed air forces open the piston-valve and escapes through the top of the barrel as before.
"The next upward movement of the piston tends to create a vacuum again, and the air in the receiver expands once more to fill it.
"This is the whole secret. The next downward movementdrives out all the air between the suction-valve and piston- valve; the following upward movement causes further expansion of the air that still remains, and so it goes on, each action of the pump removing some of the air, the rest expanding to fill the space, until that which remainsbecomes thinner and thinner-so thin and feeble, indeed, that it has not power to force open the suction-valve. We then say that the receiver has been exhausted."