heard repeated once in a clear and distinct manner, not far from us; and again, a second and a third, and even a fourth time, but as if from a progressively increasing distance, until they died away. The experiment more than once repeated, proved equally successful. We were assured that the repetitions of the sound are more numerous when the experiment is made in a boat placed midway between the two banks.

careless boatman, less watchful of his course, passes his time in calling on Lurley to repeat his "halloos." The science of acoustics, like the other physica! sciences, has been in a constant state of advancement since the revival of learning. It appears that Pythagoras knew the relation between the length of strings and the musical sounds which they produce. Aristotle was not only aware of this relation, but, likewise, that the same relation subsists between the length of pipes and their notes, and that sound was transmitted by the atmosphere. This constituted the sum of ancient learning in this branch of science. These facts were taught by Galileo, and, moreover, that the difference in the acuteness of sounds depends on the different frequency of vibrations, and that the same string, if of uniform thickness and density, must perform its vibrations in equal times. But, without attempting a history of modern discoveries in acoustics, Moreland, Newton, Daniel Bernoulli, D'Alembert, Euler, Robison, Lagrange, Laplace, Chladni, T. Young, and Biot, are all connected with it. Of these, Newton gave the law of transmission, which we have stated in this article; and the correction for heat was made by Laplace.

It is not to be expected that so remarkable and striking a phenomenon should go without being converted into an allegorical tale, during the ages of ignorance and superstition. How, in fact, was this never-failing repetition of the fisherman's choired morning prayer and evening song, which some invisible voice responded in the distant space as he glided over the bosom of the water, to or from his daily toil, to be accounted for? Imagination, ever ready to plunge into the supernatural, created a lovely we can only mention, that the names of Taylor, nymph, and placed her abode on the rock of Lurley, from the sides of which the sounds are principally reverbrated. A dangerous eddy lies on the broad shadow of this rock, and many a time, when the amazed boatman tracked his way through the stream, listening to the mysterious voice from the height of Lurley, his frail bark drawn within the vortex, would miserably perish under the rocky dwelling of the syren. Attracted by the reports of her beauty, and spurred on by the proclaimed cruelty of her disposition, the youthful son of the Count Palatine, of a neighbouring country, determined on seeing the virgin of Lurley, and carrying her a captive to his father's court. His fate was sad; for on arriving, escorted by a few followers, in the agitated waters of Lurley, his boat whirled round and disappeared. Grieved at the loss of his child, the Palatine count dispatched a trusty band to seize the relentless nymph; and just as their rude leader, unmoved by her heavenly charms and dishevelled tresses, was in the act of summoning her to surrender, a sudden hurricane swelled the stream, the waves, crested with foam, rose to the top of the rock, and encircling the lovely Undine, saved her from the rude grasp of man, and carried her to the realm of her fathers. Her voice is still heard returning the song of merriment or sorrow, but her beauteous form appears not on the heights of Lurley.

The vibrations immediately dependant on elasticity are those of rods, plates, rings, and vessels. These admit of much greater variety, and are of more difficult investigation than the vibration of chords. A rod may be either wholly loose, or fixed at one end only, or at both; and it may either be loosely fixed, in situation only, or firmly fixed, in direction as well as in situation; and these conditions may be variously combined with each other; the rod may also have a variety of secondary vibrations besides the principal or fundamental sound. All these cases have been examined by various mathematicians; the subject was begun by Daniel Bernoulli, and much extended by Euler, some of whose conclusions have been corrected by Riccati; and Chladni has compared them all with experiment. The sounds produced by the same rod, either under different circumstances, or as harmonics which may be heard at the same time, are scarcely ever related to each other in any simple proportion, except that when a rod is loosely fixed at both ends, the frequency of the vibrations of the subordinate notes is expressed by the series of the In this short legend we can trace the working of squares of the natural numbers, as 1, 4, 9, and 16. the mind, under the influence of the heart. Those But the times occupied by any similar vibrations of were not times for the march of intellect, but for that rods, similarly circumstanced, are always directly as of the passions. Hence the Age of Romance. But the squares of their lengths, and inversely as their now that the heart has lost its influence on the ac- depths. When the rod is wholly at liberty, two, at tions of men, under the management of societies for least, of its points must be at rest, and these are at diffusing useful knowledge, and of mechanical insti- the distance of about one-fifth of its length from tutes-now that the sixpenny treatises on natural either end, in the next sound of the same rod, the philosophy, on hydraulics, and acoustics, all per-middle point is at rest, with two others near the ends. spicacious and free from errors, enable the commonest understanding to explain on the simplest principles what was before a complicated phenomenon the echo of Lurley would be accounted for by the singular disposition of the two elevated banks of the river, following parallel lines in a serpentine direction-thus presenting to the rays of sound more The vibrations of rings and of vessels are nearly than one reflecting surface. This disposition of the connected with those of plates, but they are modified two banks, which are here in some parts scarcely in a manner which has not yet been sufficiently more than 1,000 feet asunder, while it accounts like-investigated. A glass or a bell divides, in general, wise for the formidable eddies which are observed in into four portions vibrating separately, and sometimes this place, explains how intricate and dangerous the into six or eight; they may readily be distinguished Lavigation must necessarily be; nay, fatal too, if the by means of the agitations excited by them in a fluid ARTS & SCIENCES.-VOL. I. B

There is by no means the same regularity in the progress of the vibrations of rods of different kinds, as in those of chords; it can only happen in particular cases, that the rod will return after a complete vibration to its original state, and these cases are probably such as seldom occur in nature.

contained in the glass. It is almost unnecessary tc | dered as composed of various vibrations in different observe, that the fluid thus applied, by adding to the planes, and which is often exceedingly complicated. mass of matter to be moved, makes the vibration These vibrations may be combined, in the first inslower and the sound more grave.

Vessels formed of glass are sometimes so arranged as to form a very pleasing musical instrument, and the editor generally uses one in his public lectures, in which the glasses are tuned by grinding their convex surfaces beneath.

stance, in a manner similar to that which has been already explained respecting the vibrations of pendulums; and if the motion of the chord be supposed to follow the same law as that of a pendulum, the result of two entire vibrations thus united, may be either a vibration in an intermediate direction, or a The arrangement of the revolution in a circle or in an ellipsis. But, besides glasses, as originally sug- these compound vibrations of the whole chord, it is gested by Dr. Arnott, is also frequently agitated by subordinate vibrations, shown in the accompany- which constitute harmonic notes of different kinds, ing figure. The small open so that the whole effect becomes very intricate; as circles represent the mouths we may observe by a microscopic inspection of any of the glasses standing in a luminous point on the surface of the chord; for inmahogany case, and the re-stance, the reflection of a candle in the coil of a fine lation of the glasses to the wire wound round it. The velocity of the motion is written musical notes, is such, that the path of the luminous point is marked shown by the common music lines and spaces which by a line of light, in the same manner as when a connect them. The learner discovers immediately burning coal is whirled round; and the figures thus that one row of the glasses produces the notes written described, are not only different at different parts of upon the lines, and the other row, the notes between the same chord, but they often pass through an the lines; and he is mentally master of the instrument amusing variety of forms during the progress of the by simple inspection. This arrangement also renders vibration; they also vary considerably according to the performance easy, for the notes most commonly the mode in which that vibration is excited. sounded in succession are contiguous: and the relations of the notes forming a simple air are so obvious to the eye, that the theory of musical combination and accompaniment is learned at the same time. The set of glasses here represented has two octaves, and the player stands at the side of the case with the notes ascending towards the right hand, as in the piano-forte.

The vibration of plates differs from those of rods in the same manner as the vibrations of membrane, differ from those of chords, the vibrations of which cause the plate to bend in different directions, being combined with each other, and sometimes occasioning singular modifications. These vibrations may be traced through wonderful varieties by Professor Chladni's method of strewing dry sand on the plates, which, when they are caused to vibrate by the operation of a bow, is collected into such lines as indicate those parts which remain either perfectly or very nearly at rest during the vibrations. Dr. Hooke had employed a similar method, for showing the nature of the vibrations of a bell, and it has sometimes been usual in military mining, to strew sand on a drum, and to judge, by the form in which it arranges itself of the quarter from which the tremors produced by countermining proceed.

A very useful instrument for ascertaining the effects of length and pressure, with reference to a vibrating string, is shown in the figure. The string is firmly attached at one extremity to the projecting arm, and passing over a bridge somewhat nearer to the centre, is strained by the weight and pulley at the opposite end. A second moveable bridge is seen near the pulley by which the length is regulated.

Under the head of organ-pipe, we purpose giving the theory of wind instruments generally; but the following collection of curious facts, bearing upon the general principles of acoustics, are too important to be omitted in our general view of the science.

tions of columns of air, has been fully examined by The resonances of sound, or reciprocated vibraMr. Wheatstone, and we cannot do better than give the result of his observations on this interesting part of acoustics. An elastic body may be made to as sume a vibratory state in two ways; either immediately, by any momentary impulse, which altering the natural positions of its particles, allows them afterwards to return by a succession of isochronous oscillations to their former state; or secondarily, by means of an immediately sounding body, which causes it to reciprocate to the latter, when certain conditions, on which depends its susceptibility of vibrating in such a manner, are fulfilled. This re

The above figures show some of the least complex | ciprocation to which, where the effect is referred to, arrangements produced by water on a glass plate-the term resonance is applied, is effected by means of the plate being put into vibration by a violin bow. If a screen be placed a few feet higher than the plate, and a strong light beneath, the figures may be rendered visible to a large audience.

It usually happens that the vibration of a cord deviates from the plane of its first direction, and becomes a rotation or revolution which may be consi

the undulations which are produced in the air, or in
any fluid or solid medium, by the periodical pulses of
the original vibrating body; these undulations being
capable of putting in motion all bodies whose pulses
are coincident with their own, and consequently with
those of the primitive sounding body. Galileo ob-
'served, that a heavy pendulum might be put in mo-

This experiment may be varied by placing the fipple of a flageolet at a short distance from the em-vibrating rods, on which vabouchure of a flute, provided of course, that the columns of air, both in the flageolet and the flute, be capable of producing the same note.

Among the Javanese musical instruments brought to England by the late Sir Stamford Raffles, there is

them to vibrate freely. Quicksilvered glass beads reflect the light of a lamp, or the sun-beams better probably than most other objects; but Mr. Wheatstone showed the editor the letters on a common address card, and which, when attached to a bent rod, produced two most elegant compound figures. The white lines beneath show the paths of a series of these rods.

already seen how capabie a conunuous pipe is of transmitting the waves or pulses of the air. This is also, to a certain extent, accomplished by a trumpetmouthed vessel, and a second apparatus may be employed to collect the pulses which have thus been transmitted.

If a sound or a wave be reflected from a curved surface, the new direction which it will assume may be determined either from the condition that the velocity with which the impulse is transmitted must remain unaltered, or from the law of reflection, which requires that the direction of the reflected pulse or wave be such as to form an angle with the surface, equal to that which the incident pulse before formed with it. Thus, if a sound or wave proceed from one focus of an ellipsis, and be reflected at its circumference, it will be directed from every part of the circumference towards the other focus; since the distance which every portion of the pulse has to pass over in the same time, in following this path, is the same, the sum of the lines drawn from the foci to any part of the curve being the same; and it may also be demonstrated that these lines form always equal angles with the curve on each side. The truth of this proposition may be easily shown by a simple experiment on a bason of water; the curvature of a circle differs so little from that of an ellipsis of small eccentricity, that if we let a drop fall into the bason near its centre, the little wave which is excited will be made to converge to a point at an equal distance on the other side of the centre. The effects of these reflections are perfectly illustrated in the accompanying diagrams.

The above figure shows at one view the joint effects of both forms of the trumpet. The organs of voice and hearing will be found under those heads.

ACRE, a measure of superficies, and the principal denomination of land-measure in use throughout the whole of Great Britain. The word (formed from the Saxon æcer, or the German acker, a field) did not originally signify a determinate quantity of land, but any open ground, especially a wide campaign; and in this antique sense it seems to be preserved in the names of places, as Castle-acre, West-acre, &c. The English standard acre, now the imperial acre of Britain, is determined from a square, cach of whose sides is the chain of 66 fect, or 22 yards, or 1-80th of a mile. Ten of these squares form the acre, which thus contains 4840 square yards. This is divided into roods, of which there are four in the acre; and into poles or perches, of which there are 40 in each rood, or 160 in the acre. The rood will thus measure 1210 square yards, and the pole 30 square yards.

The above is the standard acre of England; but various customary acres are in use throughout the different counties, deviating considerably from this standard both in excess and defect, though all of them are now illegal since the act 5 George IV., which establishes the same standard throughout the whole kingdom. In Bedfordshire, it is sometimes only two roods; Cheshire, formerly, and still in some places, 10,240 square yards; Cornwall, sometimes 5760 yards; Dorsetshire, generally 134 perches; Hampshire, from 107 to 120 perches, but sometimes 180, Herefordshire, two-thirds of a statute acre. The acre for hops contains 1000 plants, and is only equal to half a statute acre; for wood, again, it is 256 perches. Leicestershire, 2308 square yards; Lincolnshire, five roods, particularly for copyhold land; Staffordshire, nearly 2 acres; Sussex, 107, 110, 120, 130, or 212 perches; the short acre 100 or 120 perches, the forest acre 180 perches. Westmoreland, 6760 square yards, or 160 perches of 6 yards square; in some parts the Irish acre is used: Worcester, the hop acre, of 1000 stocks, 90 perches, sometimes 132 or 141 perches.

An umbrella held in a proper position over the In North Wales, the Erw or true acre is 4320 square head, may serve to collect the force of a distant yards, the Stang or customary acre 3240 square sound by reflection, in the manner of a hearing yards, as in Anglesea and Caernarvonshire, making trumpet; but its substance is too slight to reflect any 5 Llathen, 160 perches of 4 yards square, called sound very perfectly, unless the sound fall on it in a paladr: 8 acres make an ox-land, and 8 of these a very oblique direction. The whispering gallery of plough-land, in Pembrokeshire. In South Wales the St. Paul's produces an effect nearly similar by a Erw varies greatly with the perch; sometimes this continued repetition of reflections. Mr. Charles's is nine feet square, 160 perches making one stangell, paradoxical exhibition of the Invisible Girl has also and four stangells one erw of 5760 yards; sometimes been said to depend on the reflection of sound; but 10 feet square, making a quart or quarter of a llath, the deception is really performed by conveying the 40 of which make a stangell, and four stangells an sound through pipes ingeniously concealed, and erw, which is thus 7840 yards, equal to the Irish opening opposite to the mouth of the trumpet from acre; sometimes 11 feet, called bat or eglwys haw, which it seems to proceed. making the erw 9384 yards, as in Glamorganshire, The speaking and hearing horns owe their opera-one-fifth more 11,261 yards; sometimes 11 feet, tion to the reflection of sound. The reader has called a llath, 48 making a quarter cy var, and four

cyvars an erw of 11,776 yards; lastly, 12 feet, giving | adhere to gold, silver, and lead, in consequence of an erw of 10,240 yards, equal to the Staffordshire acre. chemical attraction. Water adheres to the greatest Nothing can show more clearly than the existence part of bodies, unless it is separated from their surof such numerous and useless diversities, the neces-face by oily substances, dust, flour, &c. Fluids do sity of the late act for establishing a uniform standard not form a surface perfectly horizontal in vessels to throughout Great Britain, and which only requires which they adhere so as to wet them, but rise, on the to be enforced with strictness to abolish for ever contrary, around the brim of the vessels. This is every other measure. In Scotland he acre is much proved by fluids poured into glasses, &c. Fluids, on more uniform, scarcely deviating in any part more the other hand, in vessels to which they do not adthan one per cent. from the standard. here, sink around the brim, and rise in the centre. Thus quicksilver in a glass forms a convex surface. This phenomenon of the rising and sinking of fluids becomes still more remarkable in vessels of a small diameter; wherefore capillary tubes, so called, are used for performing experiments, and the singular effects produced are ascribed to capillary attraction.

ACROTERIA, OF ACROTERS, Small pedestals usually without bases, placed in the middle and at the two extremities of pediments, and serving also to support statues. The same term was also applied to the figures placed as ornaments on the summits of public edifices.

ACTION, is that motion which one body produces or endeavours to produce in another. Mechanical action is exerted either by percussion or by pressure; and, in either case, the force exerted by the acting body is repelled in an equal degree by the body on which it acts. Thus, in driving a nail with a hammer, the stroke acts as powerfully against the face of the hammer as against the head of the nail; and in pressing the hand upon a stone, the pressure upon the stone is equally great with that upon the hand. In each of these cases the impulse is counteracted by what is termed the re-action; and that action and re-action are always equal, is not only laid down as an axiom in mechanics, but is understood to be a general law of nature.

ACTIVE FORCE, see Force.

AcTUs, a term used by Vitruvius as a measure of 100 Roman feet in length. It was also applied in agriculture to the length of one furrow, or the distance traversed by a plough in one continued line. ACUPUNCTURE, a surgical operation, which is performed by the insertion into the part affected of a silver or golden needle. It is practised to a large extent by the Chinese and Japanese, who employ it beneficially in cases of headache, lethargy, convulsion, colic, and other maladies. It has been introduced into this country, and resorted to with great advantage, particularly in hydrocephalous diseases, in which Dr. Conquest has employed it with eminent

success.

ACUTE ANGLE. See Geometry. ACUTE, in Music, is used to express the quality of a tone which is sharp or high in comparison with some other. It is thus opposed to grave.

ADDITION, the first of the four operations of Arithmetic, which see.

ADDITIONS, in Heraldry, are a species of bearings in coat armour, in which are placed rewards or additional marks of honour.

ADDUCTORS, an anatomical term, applied to those muscles which draw together the parts into which they are inserted.

ADHESION, according to the latest phraseology of physics, means generally the tendency of heterogeneous bodies to adhere gether; but cohesion implies the attraction of homogeneous particles of bodies. Adhesion may take place between two solids, as two hemispheres of glass, or between a solid and a fluid, or between two fluids, as oil and water. Thus it is said that a fluid adheres to a solid, as water to the finger dipped into it. But there is a great difference, in this respect, in different bodies; thus small particles of quicksilver do not adhere to glass, but they

Adhesion should, strictly speaking, be divided into two kinds. The one, a species of natural attraction which takes place between the surfaces of bodies, whether similar or dissimilar, and which, in a certain degree, connects them together; the other, the joining or fastening together of two or more bodies by the application of external force.

With respect to the first-mentioned, it has been proved, that the power of adhesion is proportionate to the number of touching points; and this, in solid bodies, depends upon the degree in which their surfaces are polished and compressed. The effects of this power are extremely curious, and, in many instances, astonishing.

Musschenbrock relates, that two cylinders of glass, whose diameters were not quite two inches, being heated to the same degree as boiling water, and joined together by means of melted tallow lightly put between them, adhered with a force equal to 130 lbs; that lead of the same diameter, and in similar circumstances, adhered with a force of 275 lbs.; and soft iron with a force of 300 lbs.

Martin, in the Philosophia Britannica, states, that with two leaden balls, not weighing above a pound each, nor touching upon more than one-thirtieth of a square each surface, he has lifted more than 150 lbs. weight; and that the force of adhesion between two brass planes, each four and a quarter inches in diameter, and smeared with grease or fat, was so great, that he never could meet with two men strong enough to separate them by pulling against each other. In the first experiment, the balls were scraped very finely with the edge of a sharp pen-knife, and then equally pressed together with a considerable force and a gentle turn of the hand.

We may now notice the adhesion of nails. Every carpenter is familiar with the use of the nail, and possesses a practical knowledge, more or less accurate, of the force of adhesion of different nails, and in different substances, so as to decide, without difficulty, what number, and of what length, may be sufficient to fasten together substances of various shapes, and subject to various strains. But, interesting as this subject unquestionably is, it has not been till very recently that the necessary experiments have been made to determine, 1st. the adhesive force of different nails when driven into wood of different species; 2d. the actual weight, without impulse, necessary to drive a nail a given depth; and 3d, the force required to extract the nail when so driven. The obtaining this useful knowledge was reserved for Mr. B. Bevan, a gentleman well known in the me