The Closed Manometer.

124. The CLOSED MANOMETER is shown in Fig. 86, and differs from the one just described, in having its vertical tube closed at the top. It is graduated on the principle enunciated in MARIOTTE'S law.

(124.) Describe the Closed Manometer. How is it graduated?

When the pressure in the boiler is one atmosphere, the mercury in the cistern and tube are at the same level, the tension of the steam and the elastic force of the air just balancing each other. When the pressure becomes two, three, four, &c., atmospheres, the air in the closed tube will occupy one half, one third, one fourth, &c., the space it did before, allowance being made for the weight of the mercury which is forced up into the tube. The instrument having been graduated, its use is evident. When it is desired to ascertain the tension of the steam in the boiler, the cock is turned, and the height to which the mercury ascends. in the tube, indicates the tension in atmospheres. Any number of subdivisions may be made in either of the two manometers described.

Besides these, there is a metallio manometer, invented by M. Bourdon, and known as BOURDON'S Metallic

Manometer. It is not so reliable as those described.

Fig. 86.

III. APPLICATION то PUMPS AND OTHER MACHINES.

The Air-pump.

125. An AIR-PUMP is a machine for exhausting the air from a closed space. The air-pump was invented by OTTO VON GUÉRICKE, in 1650.

A perspective view of one of the most common forms of the airpump is given in Fig. 87. The details of its construction will be best studied from Figs. 88 and 89; the former represents a longi

Illustrate. How is this manometer used? (125.) What is an Air-pump? When invented, and by whom?

tudinal, and the latter a transverse section. In all of the figures, the same letters indicate corresponding parts.

The air-pump consists of two glass cylinders, called barrels, in which are pistons, P and Q, made of leather, thoroughly soaked in oil. The pistons are attached to rods, and are elevated and depressed by a lever, NM, Fig. 89, which imparts an oscillating motion to a pinion, K. The teeth of this pinion engage with corresponding ones

Give a complete description of the air-pump. Barrels. Pistons Rods.

on the inner sides of the piston rods, A and B. The machine is so arranged that one rod ascends whilst the other descends. The cylinders rest upon and are firmly attached to a platform, H, Fig. 88. On the same platform, H. is a column, I, which supports a plate, G. Resting upon the plate G, is a bell glass, R, called a receiver. The receiver communicates with both cylinders by a pipe, shown in

This pipe branches near the cylinders, one branch leading to each cylinder, as shown in Fig. 89. The pipe communicates with the cylinders by openings, which may be closed by conical valves, a and b. The valves a and b are attached to rods which pass through the pistons, and fitted to slide with gentle friction as the pistons move up and down. In the pistons are valves, s and t, which are gently

Receiver. Pipe. Valves. Valve rods.

pressed by spiral springs so as to permit the condensed air to escape and then to close the orifices in the valves. All of the valves, a, b,

s, and t, open upwards.

In explaining the action of the air-pump, it will be suffi cient to consider a single barrel, as shown in Fig. 88. The piston, P, being at the bottom of the barrel, the valves a and t are closed. If the piston be raised, the valve a is opened, whilst the valve t is kept closed by the spiral spring and the pressure of the atmosphere. The valve a is soon arrested by its rod coming in contact with the top of the barrel, and it then remains open during the ascent of P. The air in the barrel above the piston is driven out at the opening, r, and that in the receiver and pipe expands so as to fill the receiver, pipe, and barrel. If the piston, P, be depressed, it at once closes the valve a, and compresses the air in the barrel till its elastic force becomes great enough to force open the valve t, when it escapes into the atmosphere.

Fig. 89.

By this double stroke of the piston, P, a portion of the air is exhausted from the receiver, and if a second double stroke be made, a portion of what remains may in like manner be exhausted, and so on until nearly a perfect vacuum is formed in the receiver,