The instrument is mounted on wheels for convenience of transportation. The lever E E' is worked by means of rods at right angles to the lever, so arranged that several men can apply their strength in working the pump. The action of the pump differs in no respect from that of the forcing pump; but when the instrument is worked vigorously, there is more water forced into the air vessel, the tension of the air is very much augmented, and its elastic force, thus brought into play, propels the water to a considerable distance from the mouth of the delivery pipe. It is this capacity of throwing a jet of water to a great distance, that gives to the engine its value in extinguishing fires.

A pump entirely similar to the fire engine in its construction, is often used under the name of the double action forcing pump for raising water for other purposes.

The Rotary Pump.

212. The rotary pump is a modification of the sucking and forcing pump. Its construction will be best understood from the drawing, which represents a vertical section through the axis of the sucking-pipe, and at right angles to axis of the rotary portion of the pump.

D

C

A A represents an annular ring of metal, which may be made to revolve about its axis 0. DD is a second ring of metal, concentric with the first, and forming with it an intermediate annular space. This space communicates with the sucking-pipe K, and the delivery pipe L. Four radical paddles C, are disposed so as to slide backwards and forwards through suitable openings, which are made in the

H

G

K

Fig. 177.

ring A, and which are moved around with it. G is a solid guide, firmly fastened to the end of the cylinder enclosing

the rotary apparatus, and cut as represented in the figure. EE are two springs, attached to the ring D, and acting by. their elastic force, to press the paddles firmly against the guide. These springs are of such dimensions as not to impede the flow of the water from the pipe K, and into the pipe L.

When the axis O is made to revolve, each paddle, as it reaches and passes the partition II, is pressed against the guide, but, as it moves on, it is forced, by the form of the guide, against the outer wall D. The paddle then drives the air in front of it, around, in the direction of the arrowhead, and finally expels it through the pipe L. The air behind the paddle is rarefied, and the pressure of the external air forces a column of water up the pipe. As the paddle approaches the opening to the pipe L, the paddle is pressed back by the spring E, against the guide, and an outlet into the ascending pipe L, is thus provided. After a few revolutions, the air is entirely exhausted from the pipe K. The water enters the channel B B, and is forced up the pipe L, from which it escapes by a spout at the top. The quantity of work expended in raising a volume of water to the spout, by this pump, is equal to that required to lift it through the distance from the level of the water in the cistern to the spout. This may be shown in the same manner as was explained under the head of the sucking and forcingpump. To this quantity of work, must be added the work necessary to overcome the hurtful resistances, as friction, &c.

This pump is well adapted to machine pumping, the work being very nearly uniform.

A machine, entirely similar to the rotary pump, might be constructed for exhausting foul air from mines; or, by reversing the direction of rotation, it might be made to force a supply of fresh air to the bottom of deep mines.

Besides the pumps already described, a great variety of others have been invented and used. All, however, .

depend upon some modification of the principles that have just been discussed.

The Hydrostatic Press.

213. The hydrostatic press is a machine for exerting great pressure through small spaces. It is much used in compressing seeds to obtain oil, in packing hay and bales of goods, also in raising great weights. Its construction, though requiring the use of a sucking-pump, depends upon the principle of equal pressures (Art. 154).

G

It consists essentially of two vertical cylinders, A and B, each provided with a solid piston. The cylinders communicate by means of a pipe C, whose entrance to the larger cylinder is closed by a sleeping valve E. The smaller cylinder communicates with the reservoir of water K, by a suckingpipe H, whose upper extremity is closed by the sleeping-valve D.

Fig. 178.

The smaller piston B, is worked up and down by the lever G. By working the lever G, up and down, the water is raised from the reservoir and forced into the larger cylinder A; and when the space below the piston F is filled, a force of compression is exerted upwards, which is as many times greater than that applied to the piston B, as the area of F is greater than B (Art. 154). This force may be utilized in compressing a body L, placed between the piston and the frame of the press.

Denote the area of the larger piston by P, of the smaller, by p, the pressure-applied to B, by f, and that exerted at F by F; we shall have,

If we denote the longer arm of the lever G, by L, and

the shorter arm, by l, and represent the force applied at the extremity of the longer arm, by K, we shall have from the principle of the lever (Art. 78),

Substituting this value of ƒ above, we have,

To illustrate, let the area of the larger piston be 100 square inches, that of the smaller piston 1 square inch; suppose the longer arm of the lever to be 30 inches, and the shorter arm to be 2 inches, and a force of 100 pounds to be applied at the end of the longer arm of the lever; to find the pressure exerted upon F

We have not taken into account the hurtful resistances, hence, the total pressure of 150000 pounds must be somewhat diminished.

The volume of water forced from the smaller to the larger piston, during a single descent of the piston F", will occupy in the two cylinders, spaces whose heights are inversely as the areas of the pistons. Hence, the path, over which ƒ is exerted, is to the path over which F is exerted, as P is to p. Or, denoting these paths by s and S, we have,

s: S: P: P;

or, since P p :: F: f, we shall have,

That is, the quantities of work of the power and resistance are equal, a principle which holds good in all machines.

EXAMPLES.

1. The cross-section of a sucking and forcing pump is 6 square feet, the play of the piston 3 feet, and the height of the spout, above the level of the reservoir, 50 feet. What must be the effective horse power of an engine which can impart 30 double strokes per minute, hurtful resistances being neglected?

SOLUTION.

The number of units of work required to be performed each minute, is equal to

2. In a hydrostatic press, the areas of the two pistons are, respectively, 2 and 400 square inches, and the two arms of the lever are, respectively, 1 and 20 inches. Required the pressure on the larger piston for each pound of pressure applied to the longer arm of the lever? Ans. 4000 lbs.

3. The areas of the two pistons of a hydrostatic press are, respectively, equal to 3 and 300 square inches, and the shorter arm of the lever is one inch. What must be the length of the longer arm, that a force of 1 lb. may produce a pressure of 1000 lbs. Ans. 10 inches.

The Siphon.

D D

214. The siphon is a bent tube, used for transferring a liquid from a higher to a lower level, over an intermediate elevation. The siphon consists of two branches, AB and BC, of which the outer one is the longer. To use the instrument, the tube is filled with the liquid in any manner, the end of the longer branch being stopped with the finger or a stop-cock, in which case, the pressure of the atmosphere will prevent the liquid from escaping

Fig. 179.