comes, then expect a great deal of wet, and probably high winds.

7. The unsetled motion of the mercury denotes uncertain and changeable weather.

8. You are not so strictly to observe the words engraved on the plates of the instrument (though in general it will agree with them), as the mercury's rising and falling: for if it stand at much rain, and then rise up to changeable, it presages fair weather; though not to continue so long as if the mercury had risen higher; and so, on the contrary, if the mer. cury stood at fair, and fell to changeable it presages foul wea ther; though not so much of it as if it had sunk lower.

From these observations it appears, that it is not so much the height of the mercury in the tube that indicates the weather, as the motion of it up and down: therefore, in order to form a right judgment of what weather may be expected, we ought to know, whether the mercury is actually rising or falling; and to this end the following rules are of use:

1. If the surface of the mercury be convex, standing higher in the middle of the tube than at the sides, it is generally a sign that the mercury is then rising. 2. If the surface be concave, it is then sinking. And, 3. If it be level, or rather, a little convex, the mercury is stationary.

Mercury, put into a glass tube, especially a small one, will naturally have its surface a little convex, because the particles of mercury attract one another more forcibly than they are attracted by glass.

If the glass be small, shake the tube, and if the air be grown heavier, the mercury will rise about half the tenth of an inch higher than it stood before; if it be grown lighter, it will sink as much. This proceeds from the mercury's sticking to the sides of the tube, which prevents the free motion of it until it be disengaged by the shock; and therefore, when an observation is to be made by such a tube, it ought always to be shaken first; for sometimes the mercury will not vary of its own accord, until the weather it ought to have indicated, be present.

The above-mentioned phenomena are peculiar to places lying a considerable distance from the equator; for in the torrid zone, the mercury in the barometer seldom either rises or falls much.

In Jamaica, it was observed by Sir William Beeston, that the mercury in the morning constantly stood at one degree below changeable, and at noon sunk to one degree above rain; so that the

whole scale of variation there was only three-tenths of an inch. At St. Helena, too, where Dr. Halley made his observations, he found the mercury to remain almost stationary, whatever weather happened.

CHAPTER XVIII.

ACOUSTICS.

415. ACOUSTICS signifies the doctrine of hear ing, and the art of assisting that sense by means of speaking trumpets, hearing trumpets, whispering galleries, &c.

416. When bodies move in elastic fluids, they condense that part towards which they move, at the same time that the part they recede from is rarefied. This condensation or rarefaction must produce an undulatory or vibratory motion in the fluid.

If a body, by percussion or otherwise, be put into a tremulous motion, every vibration of the body will excite a wave in the air, which will proceed in all directions, so as to form a hollow sphere; and the quicker the vibrations of the body exceed each other, the less will be the distance between each successive wave. The sensation excited in the mind by means of these waves, which enter the ear, and produce a like motion in a thin membrané stretched obliquely across the auditory passage, is called sound.

That bodies move or tremble when they produce sound, requires no particular proof: it is evident in drums, bells, and other instruments, whose vibrations being large and strong, are therefore more perceptible; and it is equally clear, that a similar vibration is excited in the air, because this vibration is communicated through the air to other bodies

that are adapted to vibrate in the same manner: thus bells, glasses, basons, and musical strings, will sound merely by the action propagated from other sounding bodies. Musicians well know that if a fiddle be played on by a good hand, and another fiddle be lying in the same room, the drawing of the bow over a particular string, will affect the similar string of the other fiddle.

417. It is established as well by mathematical reasoning, from the nature of an elastic fluid, whose compression is as the weight, as from experiment, that all sounds whatever, arrive at the ear in equal times, from sounding bodies equally distant. This common velocity is 1142 English feet in a second of time.

The knowledge of the velocity of sound, is of use in determining the distances of ships, or other objects: for instance, suppose a ship fires a gun, the sound of which is beard five seconds after the flash from the ignition of the powder is seen; then 1142 multiplied by 5, gives the distance £710 feet, or an English mile and 330 feet. 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.

418. When the aerial 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 these reflected waves, will perceive a sound similar to the original sound, but which will seem to proceed from a body situated in like position and distance behind the plane of reflection, as the real sounding body is before it. This reflected sound is called an echo.

Echoes are produced by the air which has been set in motion striking against a wall, rock, or such like, by which it is repelled. Echoes are familiar even to children, who fancy they hear the house talking to them. 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 reflect it. That an echo may take place, the repelling object must be at least thirty feet distant.

Echoes are produced by mountains on the firing of guns. 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, cœlumque tueri

Jussit, et erectos

In the lake of Killarney, in Ireland, and in the subterraneous wonders of the Peaks of Derbyshire, echoes add much to the delight of the traveller.

"At two miles from Milan," says Addison, "there stands a building that would have been a master-piece of its kind had the architect designed it for an artificial echo. We discharged a pistol, and had the sound returned to us above fifty-six times, though the air was very foggy. There are two parallel walls which beat the sound back on each other till the adulation is quite worn out."

Some buildings have a remarkable property in conveying sound. In buildings of an elliptical shape a whisper in one focus will be distinctly heard in the other focus. The concert-rooms at Edinburgh are so contrived, that the performers sit in one focus, and the audience in the other. In the whispering gallery of St. Paul's, a person speaking in the lowest tone of voice, is distinctly heard at the opposite side. A person sitting in one of the recesses of Westminster bridge over the Thames, readily hears the sound of a person speak ing in the opposite recess.

419. The waves of sound being thus reflexible, nearly the same in effect as the rays of light, may be deflected or magnified by much the same contrivances as are used in optics.

From this property of reflection, it happens that sounds uttered in one focus of an elliptical cavity, are heard much magnified in the other focus; instances of which are found in several domes and vaults, particularly the whispering-gallery of St. Paul's Cathedral, London, where a whisper uttered at one side of the dome is reflected to the other, and may be very distinctly heard. On this principle also are constructed the speaking-trumpet and the hearing-trumpet, which either are, or ought to be, hollow parabolic conoids, having a perforation at the vertex, to which the mouth is to be applied in speaking, and the ear in hearing.

420. But 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 tenison, produce a certain tone, half that length, with the same tenison, will give the octave, two thirds 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 ratio of the square roots of the weights.

421. Sound, in fact, is occasioned by the vibrations of bodies, such as musical chords, or pulses of air, which produce 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 two hundred and fifty six 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 one hundred and twenty-eight, the note is an octave below the former; if the vibrations be double, or five hundred and twelve, the note will be an octave higher. A body which gives the gravest harmonic sound vibrates twelve times and a half in one second, and the shrillest sounding body vibrates fifty-one thousand one hundred times in a second. See our chapter on Music.

422. Many amusing experiments have been made in acoustics; even in the time of Cervantes the science of acoustics was studied, as may be judged from the exquisite description of Don Antonio's oracular head, to which we now give the name of the conversing statue.