in the well; and when the piston is pushed down, the water below it not being able to go downwards, on account of the valve a, raises the valve in the piston, and gets above it; and when the piston is drawn up, it lifts all the water above it, while the pressure of the atmosphere causes more water to supply its place by raising the valve a. Thus by successive motions of the piston, the water is lifted to the top, and discharged into the head, whence it flows off by the spout. In this pump, the water raised is always equal to a column, whose base is equal to the top of the piston, and whose altitude is equal to the distance from the piston to the head. This kind of pump is much used in water-works, and like the last is very simple in its operation.

The forcing-pump is represented by fig. 7. It consists of a barrel A B, and a piston, or forcer, C. There are likewise two fixed valves, one at D, and the other at S, so disposed, as to permit the water to rise freely, but to prevent its return. When the forcer is first moved upwards in the barrel, the air between that and the water below having room to dilate, by its natural spring, will be rarefied: the water will rise in AB, and after a few strokes, fill the cavity between E and S; and as it cannot escape downwards by the lower valve at D, it will by the pressure of the piston, or plunger, C, be forced through the valve at S; that valve, which shuts of itself, being so made that the water cannot return. By every fresh stroke, more water is raised and forced into the vessel WV. This vessel is closed at top, and made air-tight by the pipe Tt, which reaches nearly to the bottom of the vessel. When the water rises in this vessel above T, the lower end of the pipe, the air which is above the water in the vessel, being now confined, will be condensed into a smaller space by the admission of more water, at each action of the piston; and pressing by its elasticity upon the surface of the water, which cannot return by the valve S, forces it up the pipe T, in a continued stream. The air vessel, must, in proportion to the other parts of the machine, be so large, that

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the change of bulk of the compressed air, during the inaction of the piston, may be inconsiderable, otherwise its action on the water will not continue steady till the next stroke.

Fire-engines, air-guns, fountains, and many other instruments, derive their efficacy from the elasticity of condensed air. The following is a description of a common engine for extinguishing houses and other buildings, when on fire. It is composed of two barrels, in each of which a solid piston is worked by means of a double lever, one piston descending and the other ascending at the same time. These barrels are fixed in a vessel of water, with which they communicate by valves, opening into them; and they also communicate with a strong vessel, by means of pipes, terminated by valves opening into it. If either of the pistons be raised, the water rushes out of the receiver through the valve; and the piston, being now depressed, forces the water into the vessel connected with the pipe by repeated strokes of the pistons, the water in that vessel condenses the air above it; the elasticity of which, by pressing upon the surface of the fluid, is sufficient to force it in a continued stream through a pipe of any length.

In an air-gun, just behind the ball, which is placed at the bottom of the barrel, is a cavity, terminated at both ends by valves opening within it; and in the stock is a condenser or forcing syringe, by which air is forced into this cavity; and when it is sufficiently condensed by repeated strokes of the piston, the valve, next the ball, is opened by the trigger, and the force of the condensed air expels the ball with a velocity proportioned to the condensation of the air. Artificial fountains are constructed on this same principle. Suppose a vessel to be constructed like the part of the air vessel, fig. 7, W X Zn, and the water filled up to T, so that the pipe Tt, may dip a certain way into it. By means of a condensing machine, which will be immediately described, air is forced in through the pipe Tt, into the water; but being lighter than that fluid, it will ascend into the part WTn; the quantity of air that may be thus thrown in, can only be limited by the strength of

the materials: When filled, and while the condensing syringe is removed from t, the stop cock a, may be turned to prevent the water from escaping till a proper jet is put on at t; when the cock a, may be again opened, and the water will rise to a great height, or may be made to pass in any form that shall be thought proper. If it is sent up in a perpendicular stream, it will be seen that its height is continually diminishing, because as the water flows out, the part W, is enlarged, aud the air, occupying a larger space, is more rare, and the pressure is much diminished.

In fig. 8, we have a representation of a condensing machine: it consists of a brass barrel containing a piston, which has a valve opening downwards; so that as the piston is raised, the air passes through the valve; but as the piston is pushed down, the air cannot return, and is, therefore, forced through a valve at the bottom of the barrel, that allows it to pass into the receiver B, but prevents it from returning. Thus at every stroke of the piston, more air is thrown into the receiver, which is made of very thick and strong glass, and the receiver is held down upon the plate C, by the cross-piece D, and the screws E and F. The air is let out of the receiver by means of the stop-cock constructed with a tube that runs to C.

If the clapper of a bell is made to strike in condensed air, the sound is much stronger than when it is struck in common air. A common glass phial exhausted of its air, and which would, from its curvature, bear the common pressure of the atmosphere will be broken to pieces by condensing the air round it.

ACOUSTICS.

WHEN bodies move in elastic fluids, they condense that part towards which they move, at the same time the part from which they recede is rarefied. This condensation and rarefaction must produce an undulatory motion in the fluid: so that if a body, by percussion, be put into a tremulous motion,

every vibration of the body will excite a wave in the air, which will proceed in all directions. The sensation excited, by waves thus formed, and which enter the ear, and produce a like motion in a thin membrane stretched across the auditory passage, is called sound. Hence it is assumed, that a sound is propagated from the sounding body, by the motion of the air. This proposition is thus proved: 1. The perception of sound, without the actual impulse of matter upon the organ of hearing, is impossible, and it must, therefore, be conveyed by some intermediate fluid. 2. The sound of a bell, included under a receiver, is weaker when the air is rarefied, and stronger when confined in condensed air. 3. A strong receiver, such as that represented in fig. 6, filled with common air, in which a bell was suspended, was fastened down in the way there represented, so that none of the included air could escape, and then covered over with a much larger receiver, and the air, contained between the two, exhausted; in this case the sound of the bell could not be heard, which proved that sound cannot be transmitted through a vacuum: the air being re-admitted between the receivers, the sound was heard. 4. That the motion is communicated by the sounding body to the contiguous air, is quite evident from the visible motion of small particles of dust floating in it; and in the vicinity of very loud sounds, as those produced by the discharge of artillery, the surface of any contiguous standing water is sensibly agitated, and even the glass in the neighbouring windows has been broken.

All sonorous bodies are elastic, which is proved by the following circumstances. 1. If glass, bells, &c. be covered with a little dust, their parts will, from the tremulous motion of the particles of the dust, appear to move when they are struck. 2. This motion is observed in water, or other fluids, contained in a glass vessel, when its edge is made to emit sound by friction. It is well known from experiment, and has been established by mathematical reasoning, that all sounds whatever arrive at the ear in equal times, from sounding bodies at equal

distances. The common velocity is at the rate of 1,142 English feet in a second of time: hence is easily ascertained the distances of ships, or other objects: thus, if a gun be fired from a ship in distress, and the report is heard at an interval of 5, 8, or 12 seconds after the flash is seen, as light, in such small distances, may be considered as instantaneous, the distance of the vessel may be considered at 5 × 1,142, or 8 × 1,142 or 12 x 1,142 feet from the observer.

When ærial waves meet with an obstacle which is hard, and of a regular surface, they are reflected; and consequently an ear placed in the course of those reflected waves, will perceive a sound similar to the original sound; but which will appear to proceed from a body situated at the same distance behind the plane of reflection, as the real sounding body is before it. This reflected sound is called an echo. From this property of reflection, it is found that sounds uttered in one focus of an ellipse, are greatly magnified in the other focus; but though the loudest echo be produced when the sounding body is in one focus of an ellipse, and the hearer in the other, yet echoes will be heard in other situations, when a sufficient number of reflected pulses arrive at the ear to excite a distinct perception. A speaker may often hear the echo of his own voice, when the reflecting obstacles are properly situated. On this subject the following proposition has been given.

If the pulses of any sound, propagated from a sounding body, or centre, A, fig. 7, strike against a number of obstacles C, D, E, &c. and the sum of the lines drawn from A to each obstacle, and from each obstacle to a second point B be equal, an echo will be heard, provided the interval AB be about 127 feet less than AC+CB. For each of the obstacles C, D, E, F, &c. will be a new centre of pulses, and one series of each will pass through B; and since by the nature of an ellipse AC÷CB; AD+DB; AE+EB, &c. are all equal to each other, the pulses propagated from A to C, D, E, &c. and thence to B, will arrive there at the same time, and concur in producing a perception of sound. Now it is asserted by