Fig. 5.
By means of a pedal made simply with two boards put together on hinges (fig. 5), one pressed with the foot, the air contained in the body of the pump (fig. 2) tends to escape. It therefore lifts the valve of the fitting fixed at A, and escapes through the flexible elastic band tied over the hole in the hollow side of tube No. 2. If the pressure ceases the big tube, on account of its own elasticity, takes its former form and sucks in the air. This time it is the valve at B which is lifted and lets pass the air which fills the body of the pump. If one has fixed on to the fitting at B, the long india-rubber tube No. 3, which is plunged in a receiver—a receiver is any vessel in which the air is exhausted, or into which it is forced—it is easily understood that after a few moves of the pedal, the air is drawn out, and a vacuum is obtained.
Fig. 6.
If one wishes to have a force-pump one has only to modify slightly the construction of the valve. Instead of a band of india-rubber fixed as shown in fig. 3, it is altered as in fig. 4, that is to say the valve is formed by a band of supple india-rubber fastened by two tacks only on one side of the opening in the side of the plug. For this object it is also necessary to take stronger tubes.
Let us now review the few experiments that can be made with this machine.
In order to conduct experiments a receiver must be obtained. The best vessel for your purpose is a large bell-jar with a ground glass stopper and neck to insure absolute tightness. Such a jar may be cheaply obtained at a scientific instrument maker’s for about seventy-five cents. If you cannot get a bell-jar procure a 4-lb. jam pot and a tightly-fitting bung. In the middle of the latter bore a hole to admit a glass tube, some six inches long and an inch in diameter, and then sealing-wax the whole of the upper surface of the bung so that air cannot enter. Over the projecting end of the glass tube, bind very tightly the free end of the long tubing affixed to the pump. To ensure tight binding, waxed thread should be used.