has a slightly acid taste, produces a disagreeable sensation on the tongue, is strongly empyreumatic, and reddens the tincture of turnsole. But it has been found, by the experiments of Fourcroy and Vauquelin, to be the acetic acid impregnated with an oil. Tartaric acid is very soluble in water. The specific gravity of a solution formed by Bergman was found to be 1.2. This solution in wa

ter is not liable to spontaneous decomposition, unless it is diluted. While it is concentrated, it loses nothing of its acid nature or its other properties. According to the analysis of Fourcroy and Vauquelin, 100 parts of this acid are composed of

Tartaric acid is not applied to any use, and but few of its combinations are employed in the practice of medicine.

TASTE, sense of. The senses of taste and smell are nearly allied to the sense of feeling; indeed they may be considered as modifications of feeling. They however are properly distinguished from it, because they have each a peculiar organ, and are each affected by peculiar properties of bodies. The chief organ of taste is the tongue; and it is fitted for its office by the numerous extremities of nerves which are lodged along its surface, and particularly at the top and sides. Hartley considers this sense as extending to the other parts of the mouth, down the throat, the stomach, and the other parts of the channel for food. Taken in this comprehensive sense, the sense of taste conveys to the mind sensations, not only of flavours, but of hunger and thirst.

In order to produce the sense of taste, the nervous extremities of the tongue must be moistened, and the action of eating generally produces an effusion of a fluid from different parts of the mouth, which answers the purpose of exciting the taste, and of assisting digestion. The pleasures derived from taste are very considerable; and the power of yielding pleasurable sensations accompanies the taste through the whole of life. Hence it is reasonable to infer, that the pleasures of taste constitute one grand source of the mental pleasures, that is, those which can be felt without the direct intervention of sensation. They leave their relics in

the mind; and these combine together, with other pleasures, and thus form feelings which often connect themselves with objects which have no immediate connection with the objects of taste. To this source Hartley traces the principal origin of the social pleasures; and there cannot be a doubt that the pleasures of taste are the chief original sources of the filial affection. It appears that one end of the long continuance of the pleasures of taste is, to supply continual accessions of vividness to the mental pleasures; but doubtless the principal object is, to make that a source of pleasure, which is necessary for self-preservation. The pains of taste are much less numerous than those of feeling. They are only such as are necessary to prompt to avoid excessive abstinence or gratification, and to prevent the employment of improper food; and therefore depend much more upon causes which man usually has under his own control.

TAUGHT, a term used in maritime business, to denote the state of being extended, or stretched out, and is usually applied in opposition to slack.

TAURUS, the bull, in zoology. See

Bos.

TAURUS, in astronomy, one of the twelve signs of the zodiac, the second in order, consisting of forty-four stars, according to Ptolemy: of forty-one, according to Tycho; and of no less than one hundred and thirty-five according to the Britannic catalogue.

white, so as to be fit for divers manufacTAWING, the art of dressing skins in tures, particularly gloves, &c. All skins may be tawed; but those chiefly used for this purpose are lambs', sheeps', kids', and goats' skins.

TAXUS, in botany, yew-tree, a genus of the Dioecia Monadelphia class and order. Natural order of Coniferæ. Essential character: male calyx none; corolla none; stamina many; anthers peltate, eight cleft; female corolla none; style none; seed one, in a berried calycle that is quite entire. There are four species; we shall notice the T. haccata, common yew-tree, which has a straight trunk, with a smooth, deciduous bark: the wood is hard, tough, and of a fine grain; leaves thickly set, linear, smooth, ever green; flowers axillary, enveloped with imbricate bractes: the male on one tree, sulphur coloured, without a calyx; the female on another, with a small green calyx, sustaining the oval flattish seed,

which calyx at length becomes red, soft, full of a sweet slimy pulp. The yew-tree is a native of Europe, North America, and Japan; its proper situation is in mountainous woods, or more particularly the clefts of high calcareous rocks. England formerly possessed great abundance, and it is now not very uncommon, in a wild state, in some parts of the country. Of planted trees there are yet several in church yards. Mr. Evelyn mentions a yew-tree in the church yard of Crowhurst, in Surrey, which was ten yards in compass; another in Braburne church-yard, not far from Scot's Hall, in Kent, being fifty-eight feet eleven inches in circumference, or nearly twenty feet in dia

meter.

TEARS, a name for the limpid fluid secreted by the lachrymal glands, and flowing on the surface of the eye; either in consequence of local irritation, or the emotions of grief. Some part of this aqueous fluid is dissipated in the air; but the greatest part, after having performed its office, is propelled by the orbicular muscle, which so closely constringes the eyelid to the ball of the eye as to leave no space between, unless in the internal angle, or where the tears are collected.

From this collection the tears are absorbed by the orifices of the punctæ lachrymale; from thence they are propelled through the lachrymal canals into the lachrymal sac, and flow through the ductus nasalis into the cavity of the nostrils, under the inferior concha nasalis. The tears have no smell, but a saltish taste. The uses of the tears are these: 1 They continually moisten the surface of the eye and eye-lids, to prevent the transparent cornea from drying and becoming opaque, or the eye from concreting with the eyelids. 2. They prevent that pain which would otherwise arise from the friction of the eye-lids against the ball of the eye from continually winking. 3. They wash away dust, or any thing acrid, that may have fallen into the eye. This liquid is transparent and colourless, has no perceptible smell, but a saline taste. It communicates to vegetable blues a permanent green colour. When it is evaporated nearly to dryness, cubic crystals are formed, which are muriate of soda. Soda is an excess, because vegetable blues are converted by it to a green colour. A portion of mucilaginous matter, which becomes yellow as it dries, remains after the evaporation. This liquid is soluble in water, and in alkalies. Alcohol produces a white flaky precipitate, and when

The mucilage of tears absorbs oxygen from the atmosphere, and becomes thick, viscid, and of a yellow colour. It is then insoluble in water. Oxymuriatic acid produces a similar effect. It is converted into muriatic acid, so that it has been deprived of its oxygen. The mucus of the nose consists of the same substance as the tears; but being more exposed to the air, it acquires a greater degree of viscidity from the mucilage absorbing oxygen.

TECHNICAL, expresses somewhat relating to arts or sciences; in this sense we say technical terms. It is also particularly applied to a kind of verses, wherein are contained the rules or precepts of any art, thus digested to help the memory to retain them; an example whereof may be seen in the article MEMORY..

TECTONA, in botany, a genus of the Pentandria Monogynia class and order. Natural order of Vitices, Jussieu. Essential character: corolla five-cleft; stigma toothed; drupe dry, spongy, within the inflated calyx; nut three-celled. There is only one species, viz. T. grandis, teak wood, or Indian oak; the trunk of this tree grows to an immense size; bark ashcoloured: branches cross-armed, numerous, spreading; young shoots four-sided; leaves opposite, above scabrous, beneath covered with soft white down; the leaves on young trees from twelve to twentyfour inches long, and from eight to sixteen broad; petiole short, thick, laterally compressed; panicle terminating, very large, cross-armed, divisions dichotomous, with a sessile fertile flower in each cleft: the whole covered with a hoary farinaceous substance; flowers small, white, very numerous, fragrant; nectary very small; nut exceedingly hard, four-celled. It is a native of the large forests in Java and Ceylon, Malabar, Coromandel, Pegu, Ava, the confines of Cochin China, and Cambodia, &c. The wood of this tree has by long experience been found to be the most useful timber in Asia; it is light, easily worked, and at the same time both strong and durable; for ship building the teak is reckoned superior to any other sort

of wood. A durable vessel of burthen cannot be built in the river of Bengal without the aid of teak plank; some of the finest merchant ships ever seen on the river Thames have arrived from Calcutta, where they were built of teak timber.

TEETH. See ANATOMY. Teeth have been analyzed by Mr. Pepys, who has found the constituent parts of teeth of different ages to be, in different proportions, phosphate of lime, carbonate of lime, and cartilage.

According to Fourcroy and Vauquelin, the enamel is composed of

TELEGRAPH, a word derived from the Greek, and which is very properly given to an instrument, by means of which information may be almost instantaneously conveyed to a considerable distance. The telegraph, though it has been generally known and used by the moderns only for a few years, is by no means a modern invention. There is reason to believe, that amongst the Greeks there was some sort of telegraph in use. The burning of Troy was certainly known in Greece very soon after it happened, and before any person had returned from thence. Now that was altogether so tedious a piece of business, that conjecture never could have supplied the place of information. A Greek play begins with a scene, in which a watchman descends from the top of a tower in Greece, and gives the information that Troy was taken. "I have been looking out these ten years (says he) to see when that would happen, and this night it is done." Of the antiquity of a mode of conveying intelligence quickly, to a great distance, this is certainly a proof. The Chinese, when they send couriers on the great canal, or when any great man travels there, make signals, by fire, from one day's journey to another, to have every thing prepared; and most of the barbarous nations used formerly to give the alarm of war by fires lighted on the hills or rising grounds.

It does not appear that the moderns had thought of such a machine as a telegraph, till the year 1663, when the Marquis of Worcester, in his "Century of Inventions," affirmed that he had discovered "a method, by which, at a window, as far

as eye can discover black from white, a man may hold discourse with his correspondent, without noise made or notice taken; being according to occasion given, or means afforded, ex re nata, and no need of provision beforehand; though much better if foreseen, and course taken by mutual consent of parties." This could be done only by means of a telegraph, which, in the next_sentence, is declared to have been rendered so perfect, that by means of it the correspondence could be carried on by night as well as by day, though as dark as pitch is black."

66

About forty years afterwards, M. Amontons proposed a new telegraph. His method was this: Let there be people placed in several stations, at such a distance from one another, that, by the help of a telescope, a man in one station may see a signal made in the next before him; he must immediately make the same signal, that it may be seen by persons in the station next after him, who are to communicate it to those in the following station, and so on. These signals may be as letters of the alphabet, or as a cypher, understood only by the two persons who are in the distant places, and not by those who make the signals. The person in the second station making the signal to the person in the third the very moment he sees it in the first, the news may be carried to the greatest distance in as little time as is necessary to make the signals in the first station. The distance of the several stations, which must be as few as possible, is measured by the reach of a telescope. Amontons tried this method in a small tract of land, before several persons of the highest rank at the court of France. It was not, however, till the French revolution, that the telegraph was applied to useful purposes.

Whether M. Chappe, who is said to have invented the telegraph first used by the French about the end of 1793, knew any thing of Amonton's invention or not, it is impossible to say; but his telegraph was constructed on principles nearly similar. The manner of using this telegraph was as follows: At the first station, which was on the roof of the palace of the Louvre at Paris, M. Chappe, the inventor, received in writing from the Committee of Public Welfare, the words to be sent to Lisle, near which the French army at that time was. An upright post was erected on the Louvre, at the top of which were two transverse arms, moveable in all directions by a single piece of

mechanism, and with inconceivable rapidity. He invented a number of positions for these arms, which stood as signs for the letters of the alphabet; and these, for the greater celerity and simplicity, he reduced in number as much as possible. The grammarian will easily conceive that sixteen signs may amply supply all the letters of the alphabet, since some letters may be omitted, not only without detriment, but with advantage. These signs, as they were arbitrary, could be changed every week; so that the sign of B for one day might be the sign of M the next; and it was only necessary that the persons at the extremities should know the key. The intermediate operators were only instructed generally in these sixteen signals; which were so distinct, so marked, so different the one from the other, that they were easily remembered.

The construction of the machine was such, that each signal was uniformly giv. en in precisely the same manner at all times. It did not depend on the operator's manual skill; and the position of the arm could never, for any one signal, be a degree higher or a degree lower, its movement being regulated mechanically. M. Chappe having received at the Louvre the sentence to be conveyed, gave a known signal to the second station, which was Mont Martre, to prepare. At each station there was a watch-tower, where telescopes were fixed, and the person on watch gave the signal of preparation which he had received, and this communicated successively through all the line, which brought them all into a state of readiness. The person at Mont Martre then received, letter by letter, the sentence from the Louvre, which he repeated with his own machine; and this was again repeated from the next height, with inconceivable rapidity, to the final station at Lisle.

Various experiments were in consequence tried upon telegraphs in this country; and one was soon after set up by government, in a chain of stations from the Admiralty office to the seacoast. It consists of six octagon boards, each of which is poised upon an axis in a frame, in such a manner that it can be either placed vertically, so as to appear with its full size to the observer at the nearest station, or it becomes invisible to him by being placed horizontally, so that the narrow edge alone is exposed, which narrow edge is from a distance invisible. Six boards make thirty-six changes, by

the most plain and simple mode of working; and they will make many more, if more were necessary: but as the real superiority of the telegraph over all other modes of making signals, consists in its making letters, we do not think that more changes than the letters of the alphabet, and the arithmetical figures, are necessary; but, on the contrary, that those who work the telegraphs should avoid communicating by words or signs agreed upon to express sentences; for that is the sure method never to become expert at sending unexpected intelligence accurately. This telegraph is, without doubt, made up of the best number of combinations possible; five boards would be insufficient, and seven would be useless. It has been objected to it, however, that its form is too clumsy to admit of its being raised to any considerable height above the building on which it stands; and that it cannot be made to change its direction, and consequently cannot be seen but from one particular point. Several other telegraphs have been proposed to remedy these defects, and perhaps others to which the instrument is still liable. The dial-plate of a clock would make an excellent telegraph, as it might exhibit one hundred and forty-four signs, so as to be visible at a great distance. A telegraph on this principle, with only six divisions instead of twelve, would be simple and cheap, and might be raised twenty or thirty feet high above the building without any difficulty: it might be supported on one post, and therefore turn round; and the contrast of colours would always be the same.

TELESCOPE, an optical instrument, which is used for discovering and viewing distant objects, either directly by glasses, or by reflection. Telescopes are either refracting or reflecting; the former consist of different lenses, through which the objects are seen by rays refracted by them to the eye; and the latter, of spečula, from which the rays are reflected and passed to the eye. The lens, or glass, turned to the object, is called the object-glass; and that next the eye, the eye-glass; and when the telescope consists of more than two lenses, all but that next the object are called eye-glasses.

The principal effects of telescopes depend upon this maxim, "that objects appear larger in proportion to the angles which they subtend at the eye and the effect is the same, whether the pencils of rays, by which objects are visible to us, come directly from the objects them

selves, or from any place nearer to the eye, where they may have been united, so as to form an image of the object; because they issue again from those points in certain directions, in the same manner as they did from the corresponding points in the objects themselves. In fact, therefore, all that is effected by a telescope is, first to make such an image of a distant object, by means of a lens or mirror, and then to give the eye some assistance for viewing that image as near as possible; so that the angle which it shall subtend at the eye may be very large, compared with the angle which the object itself would subtend in the same situation. This is done by means of an eye-glass, which so refracts the pencils of rays, as that they may afterwards be brought to their several foci, by the natural humours of the eye. But if the eye had been so formed as to be able to see the image with sufficient distinctness, at the same distance, without an eye-glass, it would appear to him as much magnified, as it does to another person who makes use of a glass for that purpose, though he would not, in all cases, have so large a field of view.

Although no image be actually formed by the foci of the pencil without the eye, yet if, by the help of an eye-glass, the pencils of rays shall enter the pupil, just as they would have done from any place without the eye, the visual angle will be the same as if an image had been actually formed in that place.

Telescopes are of several kinds, distinguished by the number and form of their lenses, or glasses, and denominated from their particular uses, &c. such are the "terrestrial, or land telescope," the "celestial, or astronomical telescope;" to which may be added, the "Galilean, or Dutch telescope," the "reflecting telescope;""the achromatic telescope,"&c. We shall proceed to describe some of these, in order to illustrate the principle. The "astronomical telescope" consists of two convex lenses, A B, K M, Plate XVI. Miscel. fig. 1. fixed at the two extremities of a tube, that consists, at least, of two parts, that slide one within the other, for adjusting the focus in proportion to the distance of the objects that are to be seen through the telescope.

PQ represents the semi-diameter of a very distant object, from every point of which rays come, so very little diverging to the object lens, K M, of the telescope, as to be nearly parallel : p q is the picture

of the object, P Q, which would be formed upon a screen situated at that place. Beyond that place, the rays of every single radiant point proceed divergingly upon another lens, A B, called the eyeglass, which is more convex than the former, and are, by this, caused to proceed parallel to one another, in which direction they enter the eye of the observer at O.

The two lenses of this telescope have a common axis, OLQ; L q is the focal distance of the object lens, and E q is the focal distance of the eye lens. An object viewed through this telescope, by an eye situated at O, will appear distinct, inverted, and magnified; viz. the object seen without the telescope will be, to its appearance through the telescope, as q E to q L; that is, as the focal distance of the eye lens to the focal distance of the object lens. For the rays, see OPTICS, which after their crossing at the place, rap, proceed divergingly, fall upon the lens, AB, in the same manner as if a real object were situated at rqp; and, of course, on the other side of that lens the rays of each pencil will proceed parallel. Now to the eye at O, the apparent magnitude of the object, or of the part PQ, is measured by the angle, EOA, or by its equal, q E p; but to the naked eye at L, when the glass is removed, the apparent magnitude of the object is measured by the angle, QLP, or by its equal, q Lp; therefore the apparent magnitude, to the naked eye, is to the apparent magnitude through the telescope, as the angle, q Lp, is to the angle, q Ep; or as the distance, 9 E, is to the distance, q L. This telescope is mostly used for astronomical observations; for, as it inverts the object, the representation of terrestrial objects through it would not be pleasant. It is evident, from the above explanation, that if the two lenses of this telescope have equal focal distances, the telescope will not magnify. It also appears, that, with a given object lens, the shorter the focus of the eye lens is, the greater will the magnifying power be. But when the disproportion of the two focal lengths is very great, then the aberration, arising from the figure of the lenses, and from the dispersive power of glass, becomes so very great as to do more damage than can be compensated by the increased magnifying power. Hence, in order to obtain a very great magnifying power, those telescopes have sometimes been made very long, as, for instance, of 100 feet, or upwards: and as they were used for astronomical pur