trates the water, on the principle of the wedge. Too great narrowness, on the other hand, is dangerous in boats that navigate stormy waters, and does not allow sufficient room for freight. To determine the shape that best combines speed, safety, and capacity, is the work of the ship-builder. It is a difficult problem, and one that is perhaps not yet solved, though great advances have been made of late years in naval architecture.

382. BARKER'S MILL.-An ingenious hydraulic machine, called Barker's Mill, and represented in Fig. 171, remains to be described.

A is an upright hollow cylinder, turning freely on a vertical axis. Through its lower end runs a horizontal tube, B C, communicating internally with the cylinder. On opposite sides of this tube, at its extremities, are two small openings. A continuous stream is introduced, through the pipe D E, into the funnel at the top of the cylinder A. It runs down into the cross-tube BC; and, if there were no opportunity of escape, it would there rest, pressing equally in every direction. The moment, however, that the two holes in the ends are opened, the water runs through; and the pressure at the holes being thus removed, while that on the opposite sides remains undiminished, the tube is forced round in the direction of the pressure, that is, in an opposite direction to the jets of water. The cylinder A turns with the tube, and thus motion is communicated to the mill-stone S. H is a hopper, which feeds the mill with grain.

Fig. 171.

BAKKER'S MILL.

383. MACHINES FOR RAISING WATER. -It is often desirable to raise water from a lower to a higher level. Well-sweeps, acting on the principle of the lever, are used for this purpose, as is also the wheel and axle in a variety of forms. But, when a large supply is required, other machines, worked with less expense of time and labor, are employed. Some of these involve the prin ciples of Pneumatics, and will be treated under that head.

the water depend? What is the advantage, and what the disadvantage, of narrowness and sharpness of prow? 382. Describe Barker's Mill, and its mode of operation. 283. What machines are used for raising water? 384. What is one of the simplest

Those that belong exclusively to Hydraulics are described. below.

384. Archimedes' Screw.-The Screw of Archimedes, called after the philosopher that invented it, is one of the simplest machines for raising water. It consists of a tube. wound spirally round a solid cylinder, as represented in Fig. 172.

To work the machine, let one end of the tube, C, rest just below the surface of the water. The cylinder, A B, must be inclined at an angle of about 35 degrees, and be fastened at the lower end in such a way as to revolve freely when turned by the handle, H. When the cylinder is turned, the open end of the tube, C, scoops up some of the water. When it has got half way round, the point D is lower than the

end C, and the water descends to D by the force of gravity. Another halfrevolution brings the point E lower than D, and again the water descends. This is continued till the water is discharged at the upper end. As new water is constantly scooped up, there will be a continuous flow as long as the handle is turned.-Archimedes' Screw operates only at short distances.

385. The Chain Pump.-The Chain Pump is much used for raising water. The principle it involves is also applied in dredging-machines, for cleaning out the channels of rivers.

This machine (see Fig. 173) consists of a continuous chain, to which circular plates, c, d, e, f, &c., are attached at equal distances. The plates are of such a size as exactly to fit the cylinder G H, the lower end of which rests in the water. The chain passes over the two wheels, I, J; to the upper one , of which, I, a handle is attached. When the handle is turned, the chain is set in motion. The plates, ascending through GH, carry up water before them, which has no opportunity of escaping till it reaches the opening K.

machines for raising water? Of what does Archimedes' Screw consist? Describe its mode of operation. At what distances does Archimedes' screw operate? 885. What machine is much used for raising water? What other application is made of the principle it involves? Describe the Chain Pump, and its mode of operating.

There it is discharged, as long as the handle is turned.

386. The Hydraulic Ram. -The Hydraulic Ram was invented in France, in 1796. It raises water by successive impulses, which have been compared to the butting of a ram, and hence its name. The re

quisite power is gained by momentarily stopping a stream in its course, and causing its momentum to act in an upward direction.

Fig. 174 represents a simple form of the Hydraulic Ram. To a stream or reservoir at A, is adapted an inclined pipe, B, through which the water that works the ram is conveyed. Near the lower end of the pipe B rises an air-chamber, D, with which an upright pipe, F, is connected. The passage connecting B with the air-chamber is commanded by a valve opening upward. At the extremity of the pipe B is another valve, E, opening downward, and made just heavy enough to fall when the water in B is at rest. Fig. 174.

E

D

The play of the valve E makes the machine selfacting. Suppose the pipe B to be filled from the reservoir; the valve E opens by its weight, and allows some of the water to escape. Soon, however, the water acquires momentum enough to raise the valve and close the open

THE HYDRAULIC RAM.

Α

ing. The stream is thus suddenly stopped, and www the pipe would be in

danger of bursting from the shock were it not for

the valve in the air-chamber D, which is at once forced upward, and allows

Why is it so called?

886. When and where was the Hydraulic Ram invented? How is the requisite power gained in the ram? Describe the hydraulic ram, and

some of the water to enter. The air in D is at first condensed by the pressure of the water thus admitted; but, immediately expanding by reason of its elasticity, it drives the water into F, for the closing of the valve prevents it from returning to B. By this time the water in B is again at rest, the valve E opens, and the whole process is repeated.

By successive impulses the water may be raised in F to a great height. A descent of four or five feet from the reservoir is sufficient. Care must be taken to have the valve E just heavy enough to fall when B is at rest, and not so heavy as to prevent it from readily rising as the momentum of the stream increases. The pipe B must also be of such length that the water, when arrested in its course, may not be thrown back on the reservoir.

387. Hydraulic Rams afford a cheap and convenient means of raising water in small quantities to great heights, wherever there is a spring or brook having a slight elevation. They are used for a variety of purposes, and particularly when a supply of water is needed for agricultural operations.

EXAMPLES FOR PRACTICE.

Friction is left out of account in these examples.

1. (See § 356, rule in italics.) Two streams issue from different orifices in the same vessel with velocities that are to each other as 1 to 6. How many times farther from the surface is the one than the other?

2. The stream A runs from an orifice in a vessel three times as fast as the stream B. How do their distances below the surface of the liquid compare?

3. In a vat full of beer there are two orifices of equal size; one 9 inches below the surface, and the other 25. How does the velocity of the latter compare with that of the former?

4. There are three apertures in a reservoir of water, 1, 4, and 16 feet below the surface. With what comparative velocity will their streams flow? 5. A stream flows from an aperture in a vessel at the rate of 4 feet in a second. I wish to have another stream from the same vessel with a velocity of 16 feet per second. How much farther below the surface than the first must it be?

6. (See § 359.) A vat full of ale, 3 feet high, has four apertures in it, 3, 12, 18, and 24 inches respectively from the top. Through which will the liquid spout to the greatest horizontal distance? Which next? Which next? 7. (See § 360.) How much water will be discharged every minute from an orifice of 3 square inches, the stream flowing at the rate of 5 feet in a second, and the vessel being kept replenished?

its mode of operating. How great a descent is required? What precautions are necessary? 887. In what case may hydraulic rams be used with advantage?

How much will be discharged every minute from another orifice in the same vessel, equally large, but situated four times as far below the surface of the liquid?

8. A stream flows from a hole in the bottom of a vessel with a velocity of 6 feet in a second. The hole has an area of 5 square inches, and the vessel is emptied in 15 seconds. How much water does the vessel hold? 9. See § 376.) A stream having a momentum equivalent to 100 units of work is applied to an Undershot Wheel; how many units of work will it perform?-Ans. 25.

(See § 377.) How many units of work will it perform, if applied to an Overshot Wheel?

(See § 378.) How many, if applied to a Breast-wheel?

(See § 379.) How many, if applied to a Turbine?

CHAPTER XII.

PNEUMATICS.

388. PNEUMATICS is the science that treats of air and the other elastic fluids, their properties, and the machines in which they are applied.

389. DIVISION OF ELASTIC FLUIDS.-The elastic fluids are divided into two classes :

I. GASES, or such as retain their elastic form under ordinary circumstances. Some of the gases, under a high degree of pressure, assume a liquid form; as, carbonic acid and chlorine; others, such as oxygen and nitrogen, can not be converted into liquids by any known process.

II. VAPORS, or elastic fluids produced by heat from liquids and solids. When cooled down, they re

sume the liquid or solid form. Steam, the vapor of water, is an example.

390. All gases and vapors have the same properties.

388. What is Pneumatics? 389. Into what two classes are elastic fluids divided? What are gases? What difference is there in the gases? What are vapors? 390. In