ed with the carnage, commanded A. to desist. Not being obeyed, he overflowed his banks, and rushed against the hero. Encouraged by Neptune and Minerva, A. opposed Xanthus, who called to his aid the waters of Simois. Juno then sent Vulcan, and the west and south winds, who drove the river-god back to his proper limits. But A. pursued the Trojans to their city, which only the interference of Apollo prevented him from taking. Hector alone remained before the Scean gate, and, having fled 3 times round the city, pursued by A., finally of fered himself for combat. A. slew him, and, after dragging his body round the city, resigned it, for a ransom, to Priam. Here the narration of Homer ends. A., as represented by this sublime poet, is of a fiery and impetuous character, and has little of that firmness and rational valor which constitute the true hero. In this respect, the heroes of the German poem "Das Nibelungenlied" are far greater and nobler than those of Homer. The further history of A. is told as follows: Falling in love with Polyxena, he sought her hand, and obtained it; for which he promised to defend Troy. But Paris slew him with an arrow, which pierced his heel, in the temple of Apollo, where he was celebrating his nuptials. Others say it was Apollo who killed him, or directed the arrow of Paris. A bloody contest ensued about his body. The Greeks sacrificed Polyxena on his tomb, in obedience to his request, that he might enjoy her company in the Elysian fields, where he is also said to have married Medea. When Alexander saw his tomb, it is said that he placed a crown upon it, exclaiming, "that A. was happy in having, during his life-time, a friend like Patroclus, and, after his death, a poet like Homer." ACHILLES TATIUS; a Greek novelist, or Erotic writer, so called, born at Alexandria, lived, probably, at the end of the 3d and the beginning of the 4th century, and taught rhetoric in his native city. In his old age, he became a convert to Christianity, and rose to the dignity of a bishop. Besides a treatise on the sphere, which we know only from an abridgment still extant, we possess a romance of his, in 8 books, styled, The Loves of Clitophon and Leucippe, which, as regards the subject and composition, is not without merit, and in some parts shows much ability. The language, though rich in rhetorical ornaments, is not free from sophistical subtilty. The charge of obscenity, which has occasionally been brought against the work, is very properly met by a Greek epigram, which remarks, that the scope of the work is to be considered, namely, to teach temperance, to show the punishment of unrestrained passions, and the reward of chastity. The best editions are the following; that published at Leyden, 1640, one published at Leipsic, by Bode, with the notes of Salmasius, 1776, and that of Mitscherlich, 1792, (Bipont.) ACHMET III, a Turkish emperor, son of Mahomet IV, reigned from 1703 to 1730. Many remarkable events took place during his reign, of which we shall here only mention, that Charles XII, after the battle at Poltawa, found protection at his court. Charles succeeded in involving A. in a war with the czar Peter the Great, which would have had a very unfortunate issue for him, if the prudence of Catharine, his mistress, whom he afterwards married, had not averted the impending danger. (See Peter I.) A. established the first printing press at Constantinople, in 1727. Towards the end of his reign, the janizaries revolted against him, and he was thrown into the same prison in which his successor, Mahomet V, had been confined, before he took A.'s place on the throne. He died in 1736. ACHMIM, Or ECHMIM; a considerable town of Upper Egypt, on the eastern bank of the Nile, called by the ancients Chemnis and Panopolis, by the Copts Smin. Though reduced from its former magnificence, it is still one of the finest towns of Upper Egypt. It has some manufactories. Abulfeda speaks of a superb temple here. The immense stones which composed it, sculptured with innumerable hieroglyphics, are now scattered about, and some are transferred into a mosque. A. contains also a triumphal arch, built by the emperor Nero. This place is famous also for the worship of the serpent Haridi. ACHROMATIC Telescopes. (See Optics.) ACIDS (acida); a class of compound bodies, which have the following characteristic properties: the greater part of them, a sour taste, and most of them are very corrosive; they change the vegetable blues to red, are soluble in water, and have great affinity for the alkaline, earthy, and metallic oxyds, with which they form neutral salts. Some acids have no sour taste, but their affinity for the three classes of bodies above-mentioned is always characteristic. If a few drops of sulphuric acid, nitric acid, or muriatic acid, be added to a solution of blue litmus, it becomes red. The same is the case if 1764, at Schneeberg, in Saxony, where they be added to other vegetable colors, his father was a saddler. He received as violet, &c. Hence these colors are his education at the Latin school of his employed as tests of acids, that is, to as- native city, and, after learning the trade certain when they exist in any substance. of his father, travelled through the counWe may add the infusion to the fluid in try as a journeyman, according to the which we are trying to detect an acid, custom of Germany. After residing for but a more convenient method is, to some time at Paris and Brussels, he went spread it on paper, and allow it to dry. to London. He there became acquaintIf a strip of this be put into a fluid in ed with Facius, a German, who had unwhich there is an acid, it instantly be- dertaken to conduct a journal of fashions, comes red. Some acids appear only in a (Journal des Modes,) and met with tolerable fluid state, either gaseous, as carbonic success. A. soon afterwards published, acid, or liquid, as sulphuric acid; others in the same way, drawings of coaches and appear in a solid form, or crystallized, as curricles, invented, drawn and painted by benzoic acid, boracic acid, &c. All acids himself. The novelty and elegance of are compound bodies, and are sometimes the forms excited universal attention, and divided into four classes, the three first of he received orders for drawings from all which are compounded with oxygen; the quarters. This laid the foundation of a fourth class consists of those which, at trade in works of art, which his activity, least according to some modern chemists, attention and precision in business so have no oxygen; e. g. sulphuretted hy- much enlarged in a short time, that he drogen. The first class consists of acids was enabled to marry an English woman, compounded with oxygen and one other became a citizen of London, and founded body; the second class comprises the an establishment called Repository of acids compounded of carbon, hydrogen Arts, in the Strand, in the centre of Lonand oxygen; the third class consists of don. It is one of the curiosities of the those acids which contain nitrogen, in British capital, and gives employment to addition to the three substances above- several hundred men. An account of mentioned. The ancient chemists were every thing new has appeared for 8 years acquainted with but few of the acids now in A.'s splendid journal, Repository of known; they divided them, according to Arts, Literature and Fashion, the first the kingdoms of nature, into mineral, series of which, in 14 volumes, costs £18; vegetable and animal acids. This divis- and the new series already amounts to ion, however, cannot now be retained, as more than 40 numbers. Every number there are some acids which appear in contains three or four elegant, colored all the kingdoms; e. g. phosphoric acid. copperplates. For 8 years he has also If the same radical be compounded with been engaged in a series of topographical different proportions of the acidifying works, exhibiting all the splendor of Britprinciple, forming different acids, the most ish aquatinta, which already constitute a powerful acid receives a name from the small library, and, for truth of design and radical, terminating in ic; the weaker, a elegance of execution, are hardly surpassname formed in the same manner in ous; ed by any similar undertaking in any e. g. sulphurous acid and sulphuric acid, country. He now has the most instructnitrous and nitric acid; and, where there ive books of the English and other lanare intermediate compounds, the term guages translated into Spanish, (princihypo is occasionally added to the com- pally by the well-known Blanco White,) pound next above it in point of acidity. and sends them to America, where his Thus hyposulphuric acid signifies an in- eldest son is engaged, in Mexico, in extermediate acid between sulphurous and tensive dealings in books and works of sulphuric acids; hypophosphorous acid, art. For some years he has also published an acid containing less oxygen than the the first souvenir in England, called the phosphorous acid. (For Prussic acid, Py- Forget me not. When the association was roligneous acid, &c. see Prussic, Pyrolig- formed, in 1813, for the relief of those neous, &c.) who had been plunged into misery by the war in Germany, A. showed himself an active philanthropist. A. is now the best lithographer in London. He employs in the summer 600 men, every day, in and around London. ACIRS; hurricanes of snow which prevail among the Cevennes, in the south of France. Villages are sometimes so rapidly covered, that the inhabitants have no means of communication, but by cutting passages under the snow. ACKERMANN, Rudolph, was born in ACOLYTHI, or ACOLYTES; servants of the church, who appeared in the Latin church as early as the 3d century; but in the Greek, not till the 5th. Their office was to light the candles, thence they were called accensores; to carry the tapers in the festal processions, thence ceroferarii; to present the wine and water at the supper; and, in general, to assist the bishops and priests in the performance of the ceremonies. They belonged to the clergy, and had a rank immediately below the subdeacons. In the Roman church, the consecration of an acolythus is the highest of the lower kinds of ordination. The person ordained receives a candlestick and chalice, in token of his ancient employment. The duties, however, formerly appertaining to this office, have been performed since the 7th century by menials and boys taken from the laity, who are improperly called acolythi, in the books of the liturgy of the Catholic church. The modern Greek church no longer retains even the name. ACONITA; a vegetable poison, recently extracted from aconitum napellus, or wolf's-bans, (properly alkaline,) by Mr. Brande. The analysis has not yet been made known. ACOUSTICS. One of our most important connexions with external objects is maintained through the sense of hearing; that is, by an affection which certain actions or motions, in those objects, produce on the mind, by being communicated to it through the ear. The peculiar excitation or motion perceptible by the ear is called sound; and the consideration of this motion, its qualities and transmission, forms the science of acoustics. Philosophers make a distinction between sound and noise: thus those actions which are confined to a single shock upon the ear, or a set of actions circumscribed within such limits as not to produce a continued sensation, are called a noise; while a succession of actions which produce a continued sensation are called a sound. It is evident from the mechanism of the ear, so far as it is understood, that it is a refined contrivance for conveying a motion from the medium which surrounds it to the auditory nerve; and that this nerve must receive every motion excited in the tympanum. Every motion thus excited, however, does not produce the sensation of sound. That motions may be audible, it is necessary that they impress themselves upon the medium which surrounds the ear with velocities comprised within certain limits. These motions are commonly produced by disturbing the equilibrium which exists be tween the parts of a body. Thus, for example, if we strike a bell, the part which receives the first impulse of the blow is driven nearer to the surrounding parts; but, the impulse having ceased, it is urged back by a force of repulsion which exists in the metal, and made to pass beyond its former position. By the operation of another property of the metal, namely, cohesive attraction, it is then made to return in the direction of its first motion, again, beyond its position of repose. Each of these agitations influences the adjacent parts, which, in turn, influence those beyond them, until the whole mass assumes a tremulous motion; that is, certain parts approach to and recede from each other; and it only recovers its former state of repose, after having performed a number of these sonorous vibrations. It is evident that such vibrations as are here described must result from the combined operation of attraction and repulsion, which, together, constitute the elasticity of solid bodies. When fluids, whose elasticity is confined to repulsion, emit sounds, a force equivalent to that of attraction in solids is supplied to them by external pressure. The sonorous vibrations of bodies are exceedingly curious, and the more difficult to be understood from our habits of measuring changes or motions by the sight; but these motions affect very sensibly another organ, while they are almost imperceptible to the eye; and, as we are without the means of converting the ideas derived from one sense into those derived from another, the sensation of the motion of sound does not assist us to understand its precise nature, as compared with visible motions. Thus, the ear at once perceives the difference between a grave and an acute sound; but it is only from attentive observation by the eye, that we discover the different rapidity of succession in the vibrations which produce them. The vibrations of a great many bodies, as strings, bells and membranes, when emitting sounds, may, however, be distinctly seen, and even felt; but they may often be rendered more sensible to the eye by a little artifice, such as sprinkling the vibrating body with sand, or some light, granular substance. Sound may be produced without vibrations or alternations; thus, if we pass the nail quickly over the teeth of a comb, the rapid succession of single shocks or noises produces all the effect of vibrations. It must be evident that the rapid motions here described, whether originating in vibrations, or a succession of concussions, must be communicated from the body, in which they are excited, to the sheet of air, or whatever else be in contact with it, and from this again to another sheet beyond the first; thus diffusing the motion in every direction. The agitation of the sounding body must thus be communicated to the surrounding medium to a great distance, and impressed upon any body situated within this distance; if this body be the ear, the tremor excited in it by these agitations will be perceived by the mind. The necessity of some medium for the transmission of sound is proved by experiment. If a bell be rung in an exhausted receiver, the sound will be hardly perceptible, while the tones will become clear and distinct, on re-admitting the air. Having thus given a general outline of the source and propagation of sound, we shall proceed to consider, with as much minuteness as the limits of this work will permit, some of the more important facts connected with them. The most obvious characteristics, by which we distinguish different sounds, consist of differences in their degrees of what we call loudness, and acuteness, or pitch. We can produce, at pleasure, sounds having different degrees of loudness, from the same sonorous body, by making the concussions upon it more or less violent; disturbing in a greater or less degree the arrangement of its parts. So two bodies of like substance and figure, but unlike mass, when subjected to the same shock, emit sounds unlike in loudness; and, again, bodies of like mass and figure, but unlike substance, form sounds more or less loud, when subjected to the same shock. In this latter case, the loudness has a relation to the quantity of elasticity possessed by the bodies; and in all cases, when the disturbance of the parts is carried beyond the elastic power of the body, so as to produce a permanent change of figure, no increase of loudness is induced. From a consideration of the preceding facts, we may conclude, that loudness depends upon the quantity of motion, or sonorous vibration, in which it originates. The other principal characteristic of sound, its acuteness or pitch, depends upon the frequency with which the concussions or vibrations of the sonorous body succeed each other. That sounds may be audible to a common ear, it is necessary that the concussions upon the medium, which communicates them, should follow each other in such succes sion, that not more than 8192, nor less than 32, distinct concussions shall be made upon the medium during the lapse of one second. Some ears, however, can perceive sounds emanating from vibrations a little beyond the extremes to which the perceptions of other ears are confined. We should be careful not to confound the frequency of vibrations with the velocity of vibratory motion. A string may vibrate with a greater or less velocity, as it passes its axis to a greater or less distance; yet the times of its vibrations may be all equal. The difference of velocity, affecting the quantity of motion only, would produce no change, except in the loudness of the sound. To those sounds which proceed from infrequent vibrations, we give the name of grave or low; those from frequent vibrations we call sharp or acute. When vibrations succeed each other in equal times, their sound excites a pleasant sensation, and they are called musical. When two bodies are made to sound together, if their vibrations are performed in equal times, the sounds are said to be in unison. When the vibrations are performed in unequal times, so that some of those of the one are not accompanied by those of the other, the ear perceives a degree of dissonance in the sounds. If, however, the vibrations meet after short and regular intervals, the dissonance is not easily detected, and the sounds are said to accord. During the continuance of most primary sounds, however excited, we perceive other and more acute sounds co-existing with them These are called their harmonics. They are supposed to originate in a series of secondary vibrations, more short and frequent than the principal vibration. Thus a sounding string, for example, may be supposed not to pass its axis in a simple curve, but to resolve itself into a tortuous line, formed by a number of smaller curves, each of which vibrates across its own axis, thus producing its harmonics. It is perhaps some combination of the harmonics with the primary sound, that characterizes the sound of different instruments, though of the same loudness and pitch, so that we can distinguish one from another. The air, being the common medium which surrounds the ear, is that by which sounds are usually transmitted. This transmission is performed with a velocity of about 1130 feet in a second All other bodies, however, are capable of transmitting sound. It may be done perfectly, even by the solid parts of the head. If, for example, we hold the stem of a watch between the teeth, and cover the ears with the hands, the beats are heard more distinctly than when the instrument is held at an equal distance in the air. The rubbing together of two stones under water may be heard, by an ear in the same medium, at the distance of half a mile. When the air, or any other body of indefinite extent, is disturbed, in a point situated within it, by a sonorous vibration, it forms a wave which passes from the disturbed point, as a centre, in every direction. It follows that as the wave extends itself, the mass to be put in motion increases until the original motion is rendered insensible from the magnitude of the mass to which it has communicated itself. The velocity with which waves, thus formed, move through any homogeneous elastic medium, is always equal to that which a heavy body would acquire by falling through half the height of the modulus of elasticity.* In applying this law to the transmission of sound by the air, it was for a long time found not to give the same results as were obtained by experiment. The discrepancy, however, has been most ingeniously reconciled by a small correction for the latent heat made sensible by the compression; the effect of this being to increase the height of the modulus of elasticity. We ought, therefore, to find that liquids, and more especially some of the solids, should transmit sound much more rapidly than air; and this agrees most perfectly with various experiments. Cast-iron, for example, has been found to transmit sound with a velocity 10 times greater than air. Sound does not readily pass from one medium to another; a sound made in the air is not easily distinguished under water, although the distance be very small. It is from this, probably, that cork and all soft cellular bodies are bad conductors of sound, as in these the sound must, in passing through the walls of the cells and the air contained in them, change successively from one medium to another. All sounds, whatever be their loudness or pitch, are transmitted with the same velocity; a fact most completely proved by every musical performance. Were it otherwise, indeed, this beautiful art could not exist. To make this apparent, it is only necessary to consider, that harmony is a combination of different sounds arranged with certain relations of time and pitch. Now, if one sound were transmitted with *The height of the modulus of elasticity of air is 4 27,800 feet. VOL. I. greater velocity than another, these relations would differ at different distances, or be confounded, except at a single given point. Nay, further; melody, which is a succession of single sounds, would not reach different ears with the same relations of time, if the different notes were not transmitted with equal velocities. Some observations on sound, in very high latitudes, seem to contradict the above law of transmission. The seeming anomaly, however, is sufficiently reconciled by supposing the different strata of air, through which the sounds, in those instances, were transmitted, in very dif ferent hygrometrical or thermometrical states; which would make corresponding differences in their modulus of elasticity. When a wave of sound meets an elastic surface, it is partly transmitted and partly reflected. This reflection, when it re turns back perpendicularly, is called an echo. That an echo may be distinctly heard, it is necessary that the reflecting surface be at such a distance that the original sound shall have ceased before the reflected one returns to the ear; otherwise they will be blended, and the echo not perceived.-Hitherto we have considered the propagation of sounds in an unconfined medium, particularly the air, in which the wave of sound can diffuse itself in every direction. When this diffitsion is prevented by enclosing the medium in a surface capable of reflecting the wave so that the sound shall be contined to one direction, the transmission from one point to another is much more perfect. Experiments have been made in this way, in which a hollow cylinder, about half a mile long, was formed by castiron pipes. The sound was transmitted by the air, in this cylinder, with wonderful distinctness. The least whisper, at one end of the cylinder, was distinctly heard at the other end. So perfect, indeed, was the transmission, “that, not to hear, it was absolutely necessary not to speak." Captain Parry and lieutenant Foster made several experiments, during the northern expeditions, to ascertain the velocity of sound. A table of them is given in a number of the Edinburgh Philosophical Journal. These experiments were made at Port Bowen, by means of a brass six-pounder, over a range of 12,892.89 feet. The results given are the mean of four shots in one case, of five in another, and, in the rest, of six shots by each observer. The mean results varied from 12,7617 to 11",7387 and 11",5311 for the time in which the range of 12,892.89 |