J. A. COOLIDGE

In our last paper we studied barometers and siphons and learned that the air exerted such a force downward that a column of mercury 30 in. high would be held up in a barometer. If we wished to make a water barometer we should need a tube over 34 ft. long. Such a barometer has been made, but is too clumsy to be of any practical value. We also find air pressure playing an indispensable part in our common lifting or water pumps, and many a column of water in a pump is held there by the pressure of air upon the water in the well.

Let us make a rude model of a pump in order to study better the working of larger pumps. A lamp chimney from a students' lamp should be fitted tightly at its small end with a disc of wood about 1/2 in. thick. In this a hole from 1/4 to 3/8 in. inches in diameter should be bored. A piece of tubing, C, glass, if possible, (a piece of metal will do. Even a piece of a "bean-blower" would answer) about 5 inches long should be fitted in A and a valve V to cover the hole should be made from a piece of leather and should be fastened by a tack in the upper part of A.

The piston B, made similarly to A but fitting more loosely into the chimney, should be attached to a rod R and should have a hole of the same size as in A with another leather valve covering this hole.

If we are content to let the water run over the top of the pump and do not care where it goes, our pump is complete. If, on the other hand, a spout is desired, one may be made in the following manner:

A collar of paper or pasteboard or several thicknesses of paper (see D in the figure) should be glued tightly to the top of the glass chimney and extend above it 1 1/2 inches. In this a spout should be fastened as in the figure. A piece of some substance, as C, about one inch long is sufficient. This collar should be thoroughly varnished or waxed, so as not to soak water. If one prefers to bore a hole through the chimney instead of making such a collar, he can do so with a rat tail file and turpentine. It is a laborious task to make a clean hole large enough, but will repay for the work done.

Experiment XXXVIII. Place the pump with tube C under water. Push down the piston B. If B fits tightly in the pump cylinder or chimney, the air in the cylinder below B and above A will be compressed and exert a strong downward pressure upon valve V, and equally strong upward pressure upon the valve in A. The result of that pressure is to shut V more tightly than ever and to push open the valve in B. The air in JET escapes. On the upward stroke of B the size of the space H is increased and the air above tries to rush in fill this space. It cannot succeed as it merely pushes down upon the valve W and closes it. the air pressing upon the water in the cistern R pushes it up through C, forces open the valve V and enters H. The next downward stroke of B causes V to shut and the water in H forces its way through W and above B. The next upward stroke lifts the water above B until it flows out through the spout D. Water from the cistern will continue to flow in through C as long as the water is lifted from H. The most difficult thing to understand is how the air pressure downward, caused by the weight of the air upon the water in R, can be transformed into an upward push of water in R and in the tube C. But as fluids exert pressure in all directions, this change of directive force is similar to the weights on the two pans of a beam balance. The air upon R balances the water in C, and as long as the tube C furnishes the opportunity the pressure on R will force the water through C until there is a balancing of weights. If everything were perfect the water would rise 34 ft. in any tube from which the air were removed. In an actual pump, because of imperfections, 24 ft. is about as high as the water will rise in the pump log.

In case our pump will not work at first because of B fitting: too loosely, a little water may be poured in the top as is often done with an actual pump by a roadside in the country.