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It is peculiar to fluids, that they do not press only in a perpendicular direction, like solids, but upwards, laterally, and in every direction. The sides of canals are on this account sometimes blown up, as it is termed; the water, by gradually washing away the earth, in time forms a kind of hollow, the pressure becomes too great for the bank to withstand, and it is forced up and falls into the canal.

The pressure of fluids is not according to their bulk, but according to their perpendicular height, combined with the area of the base. If two vessels be filled with water or any other fluid, the one of a cylindrical and the other of a conical shape, provided the bases and perpendicular heights are equal, the perpendicular pressure of the liquid will be equal: should it happen that their contents become frozen, then the pressure on the bottom of the cylindrical vessel will be three times as great as that on the bottom of the conical vessel, the volume of a cylinder being equal to three times that of a cone of the same base and height. If a long tube of only half an inch in diameter be inserted into a large cask of water and made air-tight, by pouring water into the tube the pressure will become so astonishingly great as to rend the cask asunder, as if with gunpowder. The pressure, notwithstanding the smallness of the diameter of the tube, being equal to a body of water of the height of the pipe, whose base is equal to the base of the cask. Bramah's press is made on this principle; and a column of water of half an inch in diameter is made to produce a pressure of several hundreds of tons. Various amusing experiments have been invented to exemplify this peculiar property of fluids, which are detailed in most works expressly on this science.

If a body swims in a fluid, it is known to displace as much of that fluid as is equal to its weight; if it sink or be immersed in a fluid, it will displace as much as is equal to its bulk; if it be suspended in a fluid, it will lose as much of what it weighed in air as is equal in weight to its bulk of the fluid. On this latter axiom depends that which is termed the Specific Gravity of bodies, or the relative weight of equal bulks of different bodies. Bodies are generally compared with water, and their specific gravity is usually found by weighing them in water. If a cubic

inch, or cubic foot, for instance, of any body, be twice the weight of a cubic inch or cubic foot of water, its specific gravity is said to be two, or twice that of water; and this is found by weighing the body first in air, in the common way, and then in water, and dividing the weight in air by the loss of weight in water. Thus, if a guinea be found to weigh 129 grains in air, and on its being suspended in water it weighs 7 grains lighter, it shows that a quantity of water of equal bulk with the guinea weighs 7 grains: 129 being divided by 74, the quotient will be 18 nearly, which is its Specific Gravity. If a body be lighter than water, as wood, cork, &c., it is first weighed in air, and then it is attached to some heavier body, that will cause it to sink in the water, the weight of which body has been counterbalanced in the opposite scale; these being immersed together will cause the balance to rise; then, by observing the loss of weight, and proceeding as before, the specific gravity may be obtained. Fragments of diamonds, &c., are put into a glass bucket, and suspended from the scale. For finding the specific gravity of fluids, a solid glass bubble is used.

From the near connexion between Hydrostatics and Hydraulics, many writers have considered them one science; yet it appears more systematic to treat of them separately, and to rank under the head of Hydraulics whatever is effected by the motion of water, as mills, pumps, fountains, &c.

One of the first principles of the science of Hydraulics is, that fluids rise to their own level; and thus they may be conveyed over hills and valleys, by means of pipes, to any height not greater than the level whence they flow. With this property the ancients were unacquainted, or they would not have formed those immense aqueducts, the remains of which still exist in many places. If the ancients wanted to convey water from one hill to another, they connected the hills by archways, and contrived that the water should flow over them; while we should effect the same by simply laying down a pipe or series of pipes.

*This is done by the Hydrostatic Balance, an instrument like a common balance, only one of the scales has a hook underneath to suspend a body so as to let it dip into a glass of water.

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A very great part of London is supplied with water by pipes from the reservoir at Pentonville. Jets, or Fountains, are on this principle; the water, by endeavouring to rise to its level, spouts out in the pleasing manner often witnessed. It will not rise quite as high as its source, from its being impeded by the resistance of the air, and the friction at the opening of the pipe.

If it be required to raise water above its level, it is done either with a common Sucking Pump, by removing the atmospheric pressure, or by the Forcing Pump. A column of air of the height of the atmosphere of any base, will counterbalance a column of water of the same base, and of about 32 or 33 feet high; the weight of each being alike. On this principle it is that the sucking pump acts; the air is exhausted from the bore of the pump by means of a piston, acting in a manner similar to a syringe; the water rises, and passing through a valve in the piston which will allow it to go through but not to return, it is thus made to issue from the spout of the pump.

In the Forcing Pump, the water is made to rise in a manner similar to what it does in the sucking pump and by means of a fixed valve it is also prevented from returning; but the piston being solid, unlike that of the sucking pump, by the action of the piston the water is forced through a tube contrived for that purpose, and may be thus thrown to a very considerable height. It is by means of a forcing pump, worked by a steam-engine, that the water is raised from the New River into the reservoir at Pentonville, above named. Fire-engines are on the principle of the Forcing Pump.

There is a small hydraulic instrument called a Siphon, which requires to be noticed; it is merely a bent tube, having one leg shorter than the other: its chief use is for drawing off liquors from one vessel to another. The shorter leg is immersed in the liquor to be drawn off, and by exhausting the air from the tube, which, if small, may be done by the breath, the liquor will then rise in the tube, and flow through the longer leg; or the tube may be filled with the liquor, and immersed in the vessel with both ends stopped, on removing the stoppage the liquor will flow off as before. The principle on which the Siphon acts is as

follows:-When the tube is exhausted of air, the pressure of the atmosphere forces the liquor up the shorter pipe, and as the upward atmospheric pressure on the outside is somewhat less than that on the liquor, it flows down, and will continue to do so until the vessel is emptied. If the

legs of the Siphon were equal, or if the longer leg were immersed in the liquor, the upward pressure of the atmosphere would prevent the liquor from flowing down the tube, by overcoming the perpendicular pressure.

ACOUSTICS.

Sound defined and explained-Air chief conductor of Sound-Intensity of Sound depending on the density of the media-Cause of the vibrations of a sounding body-Velocity of Sound-Conductors of Sound-Echo-Speaking and hearing Trumpets-Invisible Girl, &c.

THIS Science treats on the nature, laws, and phenomena of sound. Sound has been defined as a sensation of the mind communicated by the ear, or it is the effect of some external collision of bodies which produces a tremulous motion or vibration, and which is communicated to the mind by means of the ear. It is generally understood that the surrounding air or atmosphere is the medium of sound; but air is not the sole conductor of sound; fluids in general, and solids of all kinds will transmit it, though not to a like extent.*

That air is the chief conductor cannot but be acknowledged; and that without that or some other fluid as a medium no sound would be heard, is evident from the fact of a bell rung under an exhausted receiver being inaudible, and becoming gradually audible as the air is admitted.

The intensity of sound is found to be in proportion to the density of the air; so that it has been ascertained by experiment that sound can be heard half as far again in carbonic acid gas as in common atmospheric air, while

* It has been supposed that there is some subtle fluid, probably of an electrical nature, in the composition of bodies whose office is to transmit sound; and that bodies transmit sound in proportion to the quantity of this fluid contained in them.

in hydrogen gas it can scarcely be heard at all. On the tops of high mountains the voice is considerably less audible than in valleys; as also the report of a gun is much less in strength and more acute in tone.

Sound appears to be communicated to the air in circular undulations, similar to the small waves produced on the surface of water when a stone is thrown into it; and this is evident to the ear in the tones of a church-bell while its sounds are dying away.

When a sonorous body is struck it becomes in a state of vibration, which is communicated to the air; and Euler was of opinion that no sound making fewer vibrations than 30 in a second, or more than 7520, is distinguishable by the human ear; the former being the most grave, and the latter the most acute tone that can be possibly sounded. The vibrations of a sounding body depend on its elasticity, and are governed by certain laws: if a musical string be divided into two parts, the sound of each half will be an octave of the whole string.

The ear is evidently the most direct instrument for the reception of sound in order to convey it to the brain, which it does by means of the auditory nerve; yet the palate, teeth, and nostrils, lend their assistance; so that the deaf may often be made to hear by holding one end of a piece of metal between their teeth while the other end is in contact with the mouth of the speaker.

Sound travels in air at the rate of 1142 feet in a second, or 13 miles in a minute; and the softest whisper is transmitted as rapidly as the loudest thunder. From a knowledge of the rate of velocity of sound, the distance of a ship at sea, or of a thunder-cloud, may be easily ascertained. If the report of a gun be heard half a minute after the flash* is seen, the object will be about six miles and a half distant, and so in proportion: the same holds good in regard to the distance of a thunder-cloud. the pulse of a full-grown man in health beats seconds nearly, the number of pulsations multiplied into 1142 feet, or 380 yards, will give the distance pretty accurately. The best conductor of sound is water. On a still night,

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* Light travels so rapidly, that at the distance of a few miles it may be said to be instantaneous.

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