glass, there is still considerable friction, for in all cases the water is found to pass more rapidly in the middle of the stream than it does on the outside, where it rubs against the sides of the tube.

The sudden turns, or angles of a pipe, are also found to be a considerable obstacle to the rapid conveyance of the water, for such angles throw the fluid into eddies or currents, by which its velocity is arrested.

In practice, therefore, sudden turns are generally avoided, and where it is necessary that the pipe should change its direction, it is done by means of as large a circle as convenient.

Where it is proposed to convey a certain quantity of water to a considerable distance in pipes, there will be a great disappointment in respect to the quantity actually delivered, unless the engineer takes into account the friction, and the turnings of the pipes, and makes large allowances for these circumstances. If the quantity to be actually delivered ought to fill a two inch pipe, one of three inches will not be too great an allowance, if the water is to be conveyed to any considerable distance.

In practice, it will be found that a pipe of two inches in diameter, one hundred feet long, will discharge about five times as much water as one of one inch in diameter of the same length, and under the same pressure. This difference is accounted for, by supposing that both tubes retard the motion of the fluid, by friction, at equal distance from their inner surfaces, and consequently, that the effect of this cause is much greater in proportion, in the small tube, than in the large one.

The effect of friction in retarding the motion of fluids is perpetually illustrated in the flowing of rivers and brooks. On the side of a river, the water, especially where it is shallow, is nearly still, while in the middle of the stream it may run at the rate of five or six miles an hour. For the same reason, the water at the bottoms of rivers is much less rapid than at the surface. This is often proved by the oblique position of floating substances, which in still water would assume a vertical direction.

Why will the glass tube deliver most? What is said of the sudden turnings of a tube in retarding the motion of the fluid? How much more water will a two inch tube of a hundred feet long discharge, than a one inch tube of the same length? How is this difference accounted for? How do rivers show the effect of friction in retarding the motion of their waters?

Fig. 94.

Thus, suppose the stick of wood e, fig 94, to be loaded at one end with lead, of the same diameter as the wood, so as to make it stand upright in still water. In the current of a river, where the lower end nearly reaches the bottom, it will incline as in the figure, because the water is more rapid towards the surface than at the bottom, and hence the tendency of the upper end to move faster than the low er one, gives it an inclination forward. Machines for raising water.

The common pump, though a hydraulic machine, depends on the pressure of the atmosphere for its effect, and therefore its explanation comes properly under the article Pneumatics, where the consequences of atmospheric pressure will be illustrated.

Such machines only, as raise water without the assistance of the atmosphere, come properly under the present article. Among these, one of the most curious, as well as ancient machines, is the screw of Archimedes, and which was invented by that celebrated philosopher, two hundred years before the Christian era, and then employed for raising water and draining land in Egypt.

Fig. 95.

g

In consists of a large tube, fig. 95, coiled around a shaft of wood to keep it in place, and give it support. Both ends of

Explain fig. 94. Who is said to have been the inventor of Archimedes' screw? Explain this machine, as represented in fig. 95, and show how the water is elevated by turning it.

the tube are open, the lower one being dipped into the water to be raised, and the upper one discharging it in an intermitting stream. The shaft turns on a support at each end, that at the upper end being seen at a, the lower one being hid by the water. As the machine now stands, the lower bend of the screw is filled with water, since it is below the surface c, d. On turning it by the handle, from left to right, that part of the screw now filled with water will rise above the surface c, d, and the water having no place to escape, falls into the next lowest part of the screw at e. At the next revolution, that portion which, during the last was at e, will be elevated to g, for the lowest bend will receive another supply, which in the mean time will be transferred to e, and thus by a continuance of this motion, the water is finally elevated to the discharging orifice p. This principle is readily illustrated by winding a piece of lead tube round a walking stick, and then turning the whole with one end in a dish of water, as shown in the figure.

Fig. 96.

a

C

Instead of this method, water was sometimes raised by the ancients, by means of a drope, or bundle of ropes, as shown at fig. 96.

This mode illustrates in a very striking manner the force of friction between a solid and fluid, for it was by this force alone, that the water was supported and elevated.

The large wheel a, is supposed to stand over the well, and b, a smaller wheel, is fixed in the water. The rope is extended between the two wheels, and rises on one side in a perpendicular direction. On turning the wheel by the crank d, the water is brought up by the friction of the rope, and falling into a reservoir at the bottom of the frame which supports the wheel, is discharged at the spout d. It is evident that the motion of the wheel, and consequently that of the rope, must be very rapid, in order to raise any considerable quantity of water by this method. But when the upward velocity of the rope is eight or ten feet per second, a large quantity of water may be elevated to a considerable height by this machine.

For the different modes of applying water as a power for How may the principle of Archimedes' screw be readily illustrated? Explain in what manner water is raised by the machine represented by fig. 96.

driving mills, and other useful purposes, we must refer the reader to works on practical mechanics. There is, however, one method of turning machinery by water, invented by Dr Barker, which is strictly a philosophical, and at the same time a most curious invention, and therefore is properly introduced here.

n

Fig. 97.
d

a

This machine is called Barker's centrifugal mill, and such parts of it as are necessary to understand the principle on which it acts are represented by fig. 97.

The upright cylinder a, is a tube which has a funnel shaped mouth, for the admission of the stream of water from the pipe b. This tube is six or eight inches in diameter, and may be from ten to twenty feet long. The arms n and o, are also tubes communicating freely with the upright one, from the opposite sides of which they proceed. The shaft d, is firmly fastened to the inside of the tube, openings at the same time being left for the water to pass to the arms o and n. The lower part of the tube is solid, and turns on a point The arms are closed

resting on the block of stone or iron, c. at their ends, near which there are orifices on the sides opposite to each other, so that the water spouting from them, will fly in opposite directions. The stream from the pipe b, is regulated by a stopcock, so as to keep the tube a constantly full without overflowing.

To set this engine in motion, suppose the upright tube to be filled with water, and the arms n and o, to be given a slight impulse; the pressure of the water from the perpendicular column in the large tube will give the fluid a velocity of discharge at the ends of the arms proportionate to its height. The reaction that is produced by the flowing of the water on the points behind the discharging orifice, will continue, and increase the rotatory motion thus begun. After a few revolutions, the machine will receive an additional impulse by the centrifugal force generated in the arms, and

What is fig. 97 intended to represent? Describe this mill.

in consequence of this, a much more violent and rapid discharge of the water takes place, than would occur by the pressure of that in the upright tube alone. The centrifugal force and the force of the discharge thus acting at the same time, and each increasing the force of the other, this machine revolves with great velocity and proportionate power. The friction which it has to overcome, when compared with that of other machines, is very slight, being chiefly at the point c, where the weight of the upright tube and its contents is sustained.

By fixing a cog wheel to the shaft at d, motion may be given to any kind of machinery required.

Where the quantity of water is small, but its height considerable, this machine may be employed to great advantage, it being under such circumstances one of the most powerful engines ever invented.

PNEUMATICS.

The term Pneumatics is derived from the Greek pneuma, which signifies breath, or air. It is that science which investigates the mechanical properties of air, and other elastic fluids.

Under the article hydraulics, it was stated that fluids were of two kinds, namely, elastic and non-elastic, and that air and the gases belonged to the first kind, while water and other liquids belonged to the second.

The atmosphere which surrounds the earth, and in which we live, and a portion of which we take into our lungs at every breath, is called air, while the artificial products which possess the same mechanical properties, are called gases.

When, therefore, the word air is used, in what follows, it will be understood to mean the atmosphere which we breathe. Every hollow, crevice, or pore, in solid bodies not filled with a liquid, or some other substance, appears to be filled with air thus, a tube of any length, the bore of which is as small as it can be made, if kept open, will be filled with air; and hence, when it is said that a vessel is filled with air, it is only meant that the vessel is in its ordinary state. Indeed, this fluid finds its way into the most minute pores of all substances, and cannot be expelled and kept out of any vessel, without the assistance of the air pump, or some other mechanical means.

What is pneumatics? What is air? What is gas? What is meant when it is said that a vessel is filled with air? Is there any difficulty in expelling he air from vessels?