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stretch, and if continued, will cause it to burst with a loud report. The elasticity of the air is thus shown. If a bladder, whose neck is tied fast, be put under the receiver of an air-pump, and the external air contained in it exhausted, the small quantity of air inclosed in the bladder will, by its own elastic force, gradually expand itself, and at last burst the bladder. Air may be condensed, by artificial means, into fifty thousand times less space than it usually occupies ; thus, the air compressed in the windgun will, by its elastic force, when discharged, drive a! bullet through a board at the distance of seventy or eighty yards. The air, by its elasticity, will so far dilate or expand itself, as to take up 13,769 times more space than before.

3. Weight and pressure. The air being a heavy body, presses like other fluids, in every direction, upon whatever. is immersed in it. Thus it is found, by means of a barometer (see the next chapter) that the weight of a column of air from the surface of the earth to the top of the atmosphere, is on a medium equivalent to the pressure of a column of mercury of equal base, in the tube of the barometer, of about thirty inches in height; it therefore follows, since a cubic inch of quicksilver is found to weigh · nearly half a pound avoirdupois, that the whole thirty inches, or the weight of the atmosphere on every square inch of surface is equal to 15lb. Hence, it is computed, that the pressure of this ambient fluid on the whole sure face of the earth, is equivalent to that of a globe of lead of sixty miles in diameter. It also appears, that the preser. sure upon the human body must be very considerable ; for if the whole surface of a man's body must be supposed to contain 14 square feet, or 2016 inches, and a column of quicksilver 29; inches high and 1 inch square weighs 15lb. the whole pressure will be 30,2401b. By this enormous pressure we should undoubtedly be crushed in a

moment, if all parts of our bodies were not filled either with air, or some other elastic Auid, the spring of which is just sufficient to counterbalance the weight of the atmosphere. : • But it is just able to do this, and no more ; for if any considerable pressure be superadded, as by going into deep water, or the like, it is always severely felt, let it be ever so equable, at least when the change is made saddenly. If, on the other hand, the pressure of the air be taken off from any part of the human body, as the band for instance, when put over an open receiver, from whence the air is afterwards extracted, the weight of the external air prevails, and we feel the hand as if strongly sucked down into the glass.

4. In the exhausted receiver of an air-pump, all bodies fall with the same rapidity. If a feather and a guinea be dropped from the top, they will both reach the bottom at the same instant, because there is then no resisting medium. If a cup made of porous wood, containing mercury, be put on the receiver of an air-pump, the air being exhausted from under it, the mercury will be driven through the pores of the wood, by the external pressure of the air. Animals cannot support life in an exhausted receiver; the continuance of the vital principle varies according to the strength of the animal. Dogs, cats, rats, and mice, die in about half a minute ; a mole in one minute; wasps, bees, hornets, and grasshoppers appear dead in two minutes, and will continue, in that state, a whole day and night, but afterwards revive, upon the re-admission of the air. Earwigs, beetles, and snails, live a long time without air; and frogs will live longer without it than toads. If a person enters a diving-bell and descends into the sea, he can only live, without farther supply, as many minutes as there are gallons of air in the bell. Even a burning candle consumes the vital part of a gallon of air in a minute. If a lighted candle be covered with a receiver, and the receiver is capable of containing a gallon of air, the candle will burn a minute; and then, after having gradually decayed, it will go out. The smoke of the candle will fall to the bottom ; it generally ascends because it is lighter than the air. This evidently shows, that a constant supply of fresh air is as necessary to feed flame, as it is to support life.

5. If a bell be placed upon a cushion, and covered with a receiver, and the air-pump be shaken to make the clapper strike against the bell, the sound will be distinctly heard. But if the air be exhausted out of the receiver, and the clapper be made to strike ever so hard against the bell, it will give out no sound; hence it appears that air is absolutely necessary for the propagation of sound. (See the next chapter.)

6. The structure of the air-pump is more simple than that of the water-pump; for as air ascends by its own elasticity, its natural tendency is to separate and leave a vacuum, and all the art necessary in constructing an airpump is to prevent the external air from supplying the place of that which thus escapes. A view of the machine itself will convey a much better idea of the important purposes to which it is applied, than any description could afford. The invention of this instrument is ascribed to Otto de Guericke, the celebrated consul at Magdeburg, in the year 1654. It was much improved by Mr. Boyle, and afterwards by Dr. Hooke. Many modern improvements have been added by Messrs. Smeaton and Nairne, and the latest air-pumps are made by Haas and Hurter, Cuthbertson and Prince, each possessing peculiar advantages.

CHAP. V.-.4COUSTICS. 1. Acoustics, also called Phonics, is the doctrine or science of sounds; it has been divided into direct, refracted, and reflected, i. e. acoustics or phonics, diacoustics or diaphonics, and catacoustics or cataphonics; but these distinctions are not at present necessary or regarded. A few principles may perhaps serve to convey an idea of this interesting branch of knowledge.

2. Sound is capable of being improved both in its creation, as in speaking, singing, &c.; and also in respect to its propagation, by the position of the sonorous body. Thus, phonics may be improved by the thinness and quiescency, (or state of rest) of sound, and by placing the sonorous body, either Lear water, which has the effect of softening the sound, or on plain ground, which conveys it to a greater distance, or near a smooth wall, especially if it be cycloidal or elliptical. And hence the theory of whispering galleries.

3. Sound travels at the rate of 1142 feet in a second ; no obstacles impede its progress, and its velocity is only diminished, in a slight degree, by a contrary wind. All sounds travel at the same rate; the report of a gun, (the fire of which we may see long before we hear the sound,) and the striking of a hammer, are equally swift in their

motions; and the softest whisper, in proportion to the distance it gives, flies as rapidly as the loudest thunder. At New Gibraltar, when the watch-word of the night, All's Well, has been given by the centinel to the patrole on the ramparts, it has been heard distinctly, in a still, serene night, and the water perfectly smooth, at Old Gibraltar, a distance of about ten miles and a half. The knowledge of the velocity of sound enables us easily to ascertain the distance of a thunder cloud : for we bave merely to ascertain the interval of time which elapses between the flash of lightning and the clap of thunder, and to allow 1142 feet distance for each second; or about four and a half average beats of the pulse to a mile.

4. An echo is a reflexion of sound, striking against sóine object, in the same manner as an image is reflected in a glass. In this respect, the science of acoustics bears some analogy to that of optics; with this difference, however, that sound does not require a polished body to reAect it. The famous echo in Woodstock Park returns seventeen syllables in the day-time, when the wind is brisk; and twenty in the night-time: for then the air being denser, the vibrations become slower, and a repetition of more syllables is heard. We are also assured, that there is a much finer echo from the north side of Stepney church, in Sussex, which, in the night-time, will repeat these twenty-one syllables:-

Os homini sublime dedit, cælumque tueri

Jussit, et erectos 5. The principal use of this science is, in relation to music, to which it gives a basis on the certain principles of mathematics. Thus, if a musical string, of any length, and a certain tension, produce a certain tone, half that length, with the same tension, will give the octave, twothirds of it the fifth, and the other notes of the scale in exact proportion. The varying of the sizes of strings will produce similar effects; or if strings of the same size be extended by different weights, the tones will become more acute, in the ratio of the square roots of the weights.

6. Sound, in fact, is occasioned by the vibrations of bodies, such as musical chords, or pulses of air, which possess a certain degree of elasticity; and the perception of

sound reaches the mind, when these vibrations are transmitted to the drum of the ear by the undulations of air, or of some of the gases. When sounds are considered in relation to their gravity or acuteness, or to the production of melody, or that union of melodies which constitutes harmony, the research evidently belongs to the science of music.

All sonorous bodies which vibrate an equal number of times in a second, yield the same sound. Thus, those which vibrate 256 times in a second, sound the note C in the middle of the musical scale. If the number of vibrations, be half the above, or 128, the note is an octave below the former; if the vibrations be double, or 512, the note will be an octave higher. A body which gives the gravest harmonic sound vibrates 121 times in one second, and the shrillest sounding body vibrates 51,100 times in a second. See our chapter on Music.

7. Many amusing experiments have been made in acoustics ; tbe reader, who is desirous of further information on this subject, will find much amusement and instruction blended together in Dr. Hutton's improved edition of Ozanam's and Montucla's Recreations in Mathematics and Natural Philosophy, 4 vols. 8vo.

CHAP. VI.-METEOROLOGY. 1. THIS branch of science treats of the atmosphere; the alterations that take place in the direction of its currents or winds; of the variations in its gravity or pressure; of the changes in its temperature; of the state of the electricity which it exhibits; and lastly, as to the visible phenomena dependent upon these changes. In this invisible, elastic fluid, we live, breathe, and have our being. Without the atmosphere, no animal could exist, or be produced ; all vegetation would cease; there would be neither rain nor dews to moisten the ground; and, though we might perceive the sun and stars like bright specks, we should be in utter darkness, having no day-light, por even twilight: nor would either fire or heat exist. Nature indeed, and the constitutions and principles of matter, would be totally changed, if this fluid were wanting.

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