of the medium through which they are propelled is sufficiently altered to form an acoustic mirage. 311. It is well known that solid bodies, in general, are good conductors of sound: thus, any agitation communicated to one end of a beam is readily conveyed to the ear applied to the other end of it. The motion of a troop of cavalry is said to be perceived at a greater distance by listening with the head in contact with the ground, than by attending to the sound conveyed through the air; and we may frequently observe that some parts of the furniture of a house are a little agitated by the approach of a waggon, before we hear the noise which it immediately occasions. The velocity with which impulses are transmitted by solids is in general considerably greater than that with which they are conveyed by the air: M. Wunsch has ascertained this by direct observations on a series of deal rods closely united together, which appeared to transmit a sound instantaneously, while a sensible interval was required for its passing through the air. It appears, from experiments on the flexure of solid bodies of all kinds, that their elasticity, compared with their density, is much greater than that of the air: thus, the height of the modulus of elasticity of fir-wood is found, by means of such experiments, to be about 9,500,000, whence the velocity of an impulse conveyed through it must be 17,400 feet, or more than three miles in a second. It is obvious, therefore, that in all common experiments such a transmission must appear perfectly instantaneThere are various methods of ascertaining this velocity from the sounds produced under ifferent circumstances by the substances to be examined, and professor Chladni has in this manner compared the properties of a variety of natural and artificial productions.

ous.

312. It does not appear that any direct experiments have been made on the velocity with which an impulse is transmitted through a liquid, although it is well known that liquids are capable of conveying sound without difficulty; professor Robison informs us, for example, that he heard the sound of a bell, transmitted by water, at the distance of 1200 feet. It is, however, says Dr. Young, easy to calculate the velocity with which sound must be propagated in any liquid of which the compressibility has been measured. Mr. Canton has ascertained that the velocity of water is about 22,000 times as great as that of air; it is therefore measured by the height of a column which is in the same proportion to thirty-four feet, that is 750,000 feet, and the velocity corresponding to half this height is 4900 feet in a second. In mercury, also, it appears, from Mr. Canton's experiments, that the velocity must be nearly the same as in water; in spirit of wine a little smaller.

313. It seems probable, from various analogies, that ice has nearly the same faculty of transmission as water itself. If a heavy blow te struck against any part of the frozen surface of a large pool or lake, a person standing at a wide distance from the spot will feel, under foot, a very sensible tremor, at some considerable ime before the noise conveyed through the atCosphere has reached his ear. It is asserted

that the savage tribes who rove on the icy steppes of Tartary can readily distinguish, from afar, the approach of cavalry, by applying their head close to the frozen surface of the ground.

314. The rate with which the tremor of sound is transmitted through cast-iron, has been ascertained from actual experiment by M. Biot. This philosopher availed himself of the opportunity of the laying of a series of iron pipes, to convey water to Paris; these pipes were about eight feet each in length, connected together with small leaden rings. A bell being suspended within the cavity, at one end of the train of pipes, on striking the clapper at the same instant against the side of the bell, and against the internal surface of the pipe, two distinct sounds were successively heard by an observer stationed at the other extremity. With a train of iron pipes of 2550 feet, or nearly half a mile in length, the interval between the two sounds was found, from a mean of 200 trials, to be 2.79 seconds. But the transmission of sound through the internal column of air would have taken 2.5 seconds: which leaves fifty-nine for the rapidity of the tremor conducted through the cast-iron. From other more direct trials it was concluded that the exact interval of time, during which the sound performed its passage through the substance of the train of pipes, amounted only to 26 100th parts of a second; being ten or twelve times less than the ordinary transmission through the atmosphere.

315. It is well known that the intensity of sound is diminished by the rarefaction of the medium in which it is produced. We might, therefore, expect the sound excited in hydrogen gas would be more feeble than what it is, under similar circumstances, produced in atmospheric air of a similar specific gravity; but the difference is actually much greater.

316. A small piece of clock-work, by which bell is struck every half-minute, being placed within the receiver of an air pump, the machine was put in motion, and, after the air had been rarefied 100 times, hydrogen gas was introduced; but the sound, so far from being augmented, was, at least, as feeble as in atmospheric air of that extreme rarity, and decidedly much feebler than when formed in air of its own density, or rarefied ten times.

317. The most remarkable fact is that the admixture of hydrogen gas with atmospheric air has a predominant influence in blunting or stifling sound. If one half of the volume of atmospheric air be extracted, and hydrogen gas be admitted to fill the vacant space, the sound will then become scarcely audible.

318. But the rate of the transmission of sound is found to vary in different gases, after the inverse subduplicate ratio of their densities: thus, through carbonic gas, the communication of the tremor would be about one-third slower than ordinary; but, through the hydrogen gas, which is twelve times more elastic than common air, the flight would very nearly exceed three and a half times the usual rapidity. An admixture of this gas with the atmosphere would, therefore, greatly accelerate the transmission of sound.

319. It may be worth while observing that Mr.

Cooper has ascertained that, if hydrogen gas be breathed for a few moments, it has the curious effect of changing the voice. The effect is observed on the person speaking, immediately after leaving the vessel of hydrogen, but it soon goes off. No instance has yet occurred in which this effect on the voice has not been produced by the hydrogen. 320. By a reference to the preceding facts, it will be evident that there is scarcely any body that does not possess the power of conducting sound by the vibration of its particles, and our space will only permit of a brief notice of the experiments by Mr. Wheatstone, tending to illustrate the phenomena of polarisation.

321. I connected,' says he, 'a tuning-fork with one extremity of a straight conducting rod, the other end of which communicated with a sounding-board on causing the tuning-fork to sound, the vibrations were powerfully transmitted, but, in gradually bending the rod, the sound progressively decreased, and was scarcely perceptible when the angle was a right one. As the angle was made more acute the phenomena were produced in an inverted order: the intensity gradually increased as it had before diminished; and, when the two parts were nearly parallel, it became as powerful as in the rectilineal transmission. By multiplying the right angles in a rod, the transmission of the vibration may be completely stopped.'

322. In these experiments the axis of the oscillations of the tuning-fork should be perpendicular to the plane of the moveable angles: for, if they are parallel, they will still be transmitted. Mr. Wheatstone gives the following explanation to prove this: I placed a tuningfork perpendicularly on the side of a rectilineal rod. The vibrations were therefore communicated at right angles; when the axis of oscillations of the fork coincided with the rod the intensity of the transmitted vibrations was at its maximum. In proportion as the axis deviated from parallelism the intensity diminished; and, when it became perpendicular, the intensity was a minimum.' The phenomena of polarisation may be observed in many chorded instruments. The chords of the harp are attached to a conductor which has the same direction as the sounding-board; if any chord be altered from its quiescent position, so that its axis of oscillation shall be parallel with the bridge or conductor, its tone will be full; but if the oscillations be excited, so that their axes shall be at right angles with the conductor, the tone will be feeble.

323. In many cases the reflection of sound becomes extremely inconvenient, by the new direction that is given to the voice in large rooms, and a variety of architectural arrangements have been employed to remedy this effect; but there is another circumstance connected with the transmission of sound in apartments, which should not escape our attention: namely, that the aerial pulses are facilitated or retarded in their progress by the artificial currents that arise from the process of ventilation.

324. The methods by which it is proposed by Mr. Matthews to counteract this acoustical defeet are as follows:-where there is room a circle should be preferred, or at least a form, as VOL. XVII.

nearly so as can be; this, however, is not positively essential. The ceiling should be one entire inverted dome, or obtuse cone, with the base upwards, extending in that direction to within about ten feet of the walls of the building. From the highest part, it should again descend to the wall, and nearly as low as the heads of the occupants of the back seats. The lowest point of the ceiling, if a circle, to be in the centre not more than fifteen or sixteen feet from the ground. The station of the speaker should be under the lowest point of the cone, with a hollow sounding-board about a foot above his head, and so constructed as to convey a clear sound behind him. A constant breeze of air will be made to pour forth in every direction, from holes in its sides, until it reaches the highest part of the ceiling. From thence it cannot return; but will be drawn off by means of ventilators that will not admit cold air: but will leave the vacuous space to be supplied from the sounding-board only. In courts of justice, and other places where the voice will have occasionally to proceed from different parts of the building, it can be made to proceed from the place where a person is speaking; and the moment he has done, to proceed from an opposite place in which another person also speaks, by opening and shutting dampers provided for this purpose. This air in winter can be heated to the degree required. Thus, the sound, instead of having occasion to cross the ascending evaporation, and be opposed to the current of air, which now rushes forward to supply its place, will pass along with the current, and a moderately lowtoned voice will be distinctly heard in every part of a large room.

325. Dr. Wollaston has lately discovered the very singular fact, that there are many persons who never felt any defect in their hearing, and who yet cannot hear certain sounds, which others perceive distinctly.

326. It is well known that persons affected with slight deafness hear sharp sounds much better than those which are grave and low. They distinguish the voices of women and children, from their acuteness, much better than the lower tones of men's voices. This fact is acted upon practically, as it may be remarked that those accustomed to speak to deaf people use a shriller tone of voice, rather than merely a louder tone than common.

327. This partial deafness may be artificially produced, by shutting the mouth and nose, and exhausting the air in the Eustachian tube, by a fercible attempt to take breath by expanding the chest. When this is carefully done, so that the exhaustion of the air behind the drum of the ear is as complete as possible, the external air is felt strongly, and even painfully, pressing on the drum; and the ear becomes insensible to low sounds, though shrill sounds are as readily perceived as before.

328. After the ear is brought into this state, it will remain so for some time, without continuing the painful effort to take breath, and even without stopping the breath.; for, by suddenly discontinuing the effort, the end of the tube will close like a valve, and prevent the air from get20

ting into the drum. The act of swallowing will
open the closed tube, and restore the air to its
wonted feeling.

329. When the ear is thus exhausted, if we at-
tempt to listen to the sound of a carriage passing
in the street, the rumbling noise cannot be heard,
though the rattle of a chain or loose screw re-
mains as easily heard as before. At a concert
the experiment has a singular effect: as none of
the sharper sounds are lost, and the great mass of
the louder sounds are suppressed, the shriller
ones are consequently so much the more dis-
tinctly heard, even to the rattling of the keys of a
bad instrument, or the scraping of catgut unskil-
fully touched.

330. Dr. Robison found, by the most distinct
experiments, that any noise whatever, will have
the effect of producing a musical note if repeated
with due frequency, not less than thirty or forty
times in a second. Nothing surely can have less

ACOUSTICS, 254.

AIR pump, described, 25.

ANIMAL body, how supported, 10.
ATMOSPHERE its height, 2.

BACCHUS apparatus, 51.

BALLOON of glass, 52.

pretension to the name of a musical sound than
the solitary snap which a quill makes when
drawn from one tooth of a comb to another: but
when the quill is held to the teeth of the wheel,
whirling at such a rate that 720 teeth pass under
it in a second, the sound of G in alt. is heard
most distinctly; and if the weight of the wheel's
motion be varied in any proportion, the noise
made by the quill is mixed, in the most distinct
manner, with the musical note corresponding to
the frequency of the snaps. The kind of the
original noise determines the kind of the con-
tinuous sound produced by it, making it harsh
and unpleasant, or smooth and harmonious, ac-
cording as the original noise is abrupt or gra-
dual: but even the most abrupt noise produces
a tolerably smooth sound when sufficiently fre-
quent. Nothing can be more abrupt than the
snap just now mentioned; yet the G produced
by it has the smoothness of a bird's chirrup.

LONDON, atmosphere of, 140-
LUNGS-GLASS, 51.

MATERIALITY of air, 48.

METEOROLOGICAL Journal, 238.
MOLECULAR vibrations, 285.
MONSOONS, 220.

MOTION of aeriform fluids, 118.

NAIRNE, Mr., experiments by, 41
NODAL lines, 262.

PAPIN, Dr., experiments by, 72.
PNEUMATICS, nature of the science, 1.
PNEUMATIC water-pump, 54.
POLARISATION of sound, 294.

PRESSURE on the bodies of animals, 11.

PROCESS of exhaustion, 36.

RAREFACTION, 39.

RECTILINEAR transmission of sound, 290.
ROD, vibration of, 262.

SAFETY-VALVE, 116.
SOUND, 254.

SUN rarefies the air, 204.

TEMPERATURE at which fluids boil, 5.
TIDE in the atmosphere, 192.
TRADE wind, 218.

TRANSMISSION of sound, 290.
TUNING forks, 298.

VELOCITY of the wind, 213.
VENTILATION, its theory, 130.
VIBRATION of rod, 262.

VOLCANOES produce winds, 196.

WARMTH communicated to air, 157.

WEIGHT of atmosphere, 8.

WELTER and Gay Lussac, their experiments, 16.

WINDOWS employed as ventilators, 149.

WINDS, on the theory of, 188.

WHEATSTONE'S experiments, 254.

PNEUMONIA. From Vevμa, breath. Pneumonitis: peripneumonia; peripneumonia vera. Inflammation of the lungs. A genus of disease in the class pyrexia, and order phlegmasia, of Cullen; characterised by pyrexia, difficult respiration, cough, and a sense of weight and pain in the thorax. The species, according to the above nosologist, are, 1. Peripneumonia. The pulse not always hard, but sometimes soft: an obtuse pain in the breast: the respiration always difficult; sometimes the patient cannot breathe, unless in an upright posture; the face swelled, and of a livid color; the cough for the most part with expectoration, frequently bloody. 2. Pleuritis. The pulse hard: a pungent pain in one side; aggravated during the time of inspiration; an uneasiness when lying on one side a very painful cough, dry in the beginning of the disease, afterwards with expectoration, and frequently bloody. See MEDICINE, Index.

PO, a large and celebrated river of Italy, anciently called Eridanus and Padus, which rises from Mount Viso, in Piedmont, seven miles north of Chateau Dauphin, on the borders of the late province of Dauphiny; crosses Saluzzo, runs through Piedmont, Montferrat, Milan, Mantua, Modena, and Ferrara; where it begins to divide at Ficheruolo, and at last falls into the Adriatic by four principal mouths. In its course it passes Ville Franca, Carmagnola, Carignano, Moncalier, Turin, Verrua, Brema, Valencia, Placentia, Cremona, Viadana, St. Benedict, Ferrara, Ariano, Comacchio, &c. It receives the waters of many Alpine rivers, which make it often overflow its banks. The principal tributary streams are the Dora Riparia, the Dora Baltea, the Stura, the Orco, the Sesia, the Tanaro, the Ticino, the Adda, the Olona, the Oglio, the Mincio, the Crostolo, and the Panaro.

POA, meadow-grass: a genus of the digynia order, and pentandria class of plants; natural order fourth, gramina: CAL. is bivalved and multiflorous: the spicula or partial spike is ovate, with the valvules scabrious and a little sharp, or thin on the margin. There are twenty species; most of them grasses, and very agreeable food for cattle. P. aquatica, water reedgrass, grows in marshes. The cattle will frequently go so deep as to endanger their lives for it. It is the largest of the British grasses, grow. ing to the height of five or six feet. The leaves are smooth, and half an inch wide or more. The panicle is eight or ten inches long, greatly branched, and decked with numerous spiculæ : these are of a reddish brown color intermixed with green, of a compressed lanceolate form, imbricated with about six flowers for the most part, but varying from five to ten.

POACH', v. a. & v. n. Į Fr. pocher, poche POACH'ER, n. s. (a pouch or pocket). To boil or parboil eggs by throwing them out of the shell into water, by which they are formed into globules or pouches.'-Thomson. Hence to parboil an undertaking, i. e. begin without completing it; see the extracts from Bacon: to steal, i. e. carry off, game (in a bag or pouch); plunder by stealth: a poacher is one who steals

game.

The yolks of eggs are so well prepared for nou

POCK, n. s. POCKHOLES, POCKY, adj.

Sax. poc, from Pox, which see. A pustule raised by the small-pox, or any similar disorder: pockholes, the holes left by pock. Are these but warts and pockholes in the face O'the' earth? Donne. POCKET, n. s. & v. a. Sax. pocca; Fr. POCK'ET-BOOK, n. s. pochette. The small POCKET-GLASS. Sbag or pouch inserted into clothes; a certain quantity of hops: to put into the pocket, taking up after it, occasionally a pocket-book and pocket-glass, mean respectively a book and glass carried for convenience in the pocket.