poses, or mostly in the night time, they were frequently used without a tube, viz. the object lens was fixed on the top of a pole, in a frame capable of motion in any required direction, and the eye lens was fixed in a short tube that was held in the hand of the observer. The distance, as well as the direction, of the two lenses were adjusted by a strong cord stretched between the frame of the object lens and the tube of the eye lens. In this construction, the instrument has been called an "aërial telescope." Its use is evidently incommodious; but it was with such a telescope that five satellites of Saturn, and other remarkable objects, were discovered.

The object, which appears inverted through this telescope, will appear upright and distinct, if two more convex eye glasses be subjoined to it, at a distance from each other, which is equal to the sum of their focal distances; and when their local distances are equal, the object will be magnified as much as without those additional glasses; but through them it will appear upright, and not inverted. Hence this telescope has been mostly used for viewing terrestrial objects, and is therefore called the "terrestrial telescope."

The "Galilean telescope" consists of a convex object lens, and a concave eye lens, and derives its name from the great Galileo, who is generally reckoned the inventor of it. Fig. 2 shows, that the distance between the two lenses is less than the focal distance of the object lens; viz. instead of the convex lens situated behind the place of the image, to make the rays of each pencil proceed in a parallel direction to the eye, here a concave eye lens is placed as much before that image: and this lens opens the rays of each pencil that converged to q and p, and makes them emerge parallel towards the eye; as is evident by conceiving the rays to go back again through the eye lens, whose focal distance is E q.

The eye must be placed close to the concave lens, in order to receive as many pencils as possible; and then supposing an emerging ray of an oblique pencil to be produced backwards along A O, the apparent magnitude of the object is measured by the angle, A O E, or its equal, q Ep, which is to the angle, q Lp, or QLP, as q L to q E, viz. as in the astronomical telescope. It is evident that in this telescope the objects appear erect, for the rays of light do not cross each other.

The field of view, or quantity of objects that are taken in at once in this telescope, does not depend upon the breadth of the eye lens, as in the astronomical telescope, but upon the breadth of the pupil of the eye : because the pupil is less than the eye lens, A B, and the lateral pencils do not now converge to, but diverge from, the axis of the lenses. Upon this account the view is narrower in this than in the preceding telescope; yet the objects through it appear remarkably clear and distinct.

"The night telescope" is a short telescope, viz. about two feet long, which represents the objects inverted, much enlightened, but not much magnified. Its field of view is also very extensive. This telescope, in consequence of those properties, is used at night mostly by navigators, for the purpose of discovering objects that are not very distant, but which cannot otherwise be seen, for want of sufficient light; such as vessels, coasts, rocks, &c. On account of its extensive field and great light, this telescope has also been advantageously used, by astronomers, for discovering some celestial objects, whose situation was not exactly known, or for viewing at once the relative situation of several stars and other objects.

This telescope has a pretty large and simple object lens, whence it derives its great light; for as the rays which proceed from every single point of the object fall upon the whole lens of a telesope, and are thence refracted to a focus, it is evident that the larger that lens is, the greater number of rays will be thrown upon that focus, and of course the brighter will the image be. In this telescope, large lenses may be used, because the telescope is not intended to magnify more than about four or six times in lineal extension.

Within this telescope a second lens is often used for shortening the focal length of the object lens. The eye lens is sometimes single, but mostly double, (viz. a combination of two plano-convex lenses placed at a little distance from each other) and pretty large: hence is derived the extensive field of view, which in some of those telescopes exceeds six or seven degrees.

We may observe, once for all, that in every telescope the distance between the object lens and the other lens or lenses must be alterable, in order that the focus may be adjusted according to the distance of the objects. Hence, every telescope

consists at least of two tubes, one of which, viz. that with the eye lenses, slides within the other. To the same telescope several eye tubes, with a shallower or deeper lens, or with a different number of lenses, may be adapted successively, in order to give them different magnify ing powers, suitable to the clearness of the air, of the objects, &c. as also for converting them into astronomical or terrestrial telescopes.

We now proceed to the reflected telescope, which is likewise called the Newtonian telescope; for, if not the original projector, Sir Isaac Newton is, at least, the first person who executed a telescope of this sort, which consists of reflecting and refracting parts.

The general principle of this telescope is the same as that of the dioptric or refracting telescope. In the latter, the rays which come from a distant object are, by the action of the convex object lens, collected to a focus, and beyond that focus the rays of every single radiant point are rendered again parallel by the action of the eye lens or eye lenses. This is other wise expressed, by saying that the object lens forms an image of the object, which image is viewed by the eye lens. In the former, viz. in the reflecting telescope, the rays which come from a distant object are, by the action of a concave reflector, sent back convergingly to a focus, where they form an image, which is viewed through the eye lens. There are several varieties of this telescope; we shall content ourselves with the description of one only, viz. the Gregorian telescope, which is represented in fig. 3. The large concave speculum, B E, of this telescope, is perforated with a hole quite through its middle. Within the tube of the telescope, a small concave speculum, xy, is supported by the arm, H, directly facing the large speculum, B E. Two lenses, wr and no, are contained in the eye tube, and the observer applies his eye to a small hole at P, in order to view a magnified distant object.

The large reflector, B E, receives the rays, a c, bd, from the distant object, and reflects them to its focus, e, where they form the inverted image, or where they cross each other, and then fall divergingly upon the small reflector, xy, whose focus is at f; viz. a little further than the focus e, of the large reflector: hence the rays are reflected back upon the lens, , not in a parallel, but in a converging manner; and that convergency is increased by the action of that lens, so as to VOL. XII.

come to a focus, or to form a second image, R S, much larger than the former, and erect like the object. Lastly, this image is viewed through the eye lens, no; or, in other words, the rays from every single point of the object, after this second crossing, fali divergingly upon the eye lens, which sends them nearly parallel to the eye at P, through a very small hole. Sometimes the eye lens, no, is double, viz. it consists of two lenses, which perform the office of a single lens.

If the first lens wx, were removed, the image would be formed somewhat larger at z; but the area or field of view would be smaller and less pleasant. At the place of the image, R S, there is situated a circular piece of brass, called a diaphragm, with a hole of a proper size to circumscribe the image, and to cut off all superfluous or extraneous light, in order that the object may appear as distinct as possible.

The magnifying power of this telescope is computed in the following manner: If this telescope consisted of the two reflectors only, and these were situated so that e were the focus of each reflector; then the rays which came parallel from the distant object to the large reflector, and divergingly from that to the small reflector, would, after the second reflection, go parallel to the eye at P, and of course the object would appear magnified in the proportion of the focal distance of the large reflector to the focal distance of the small reflector; so that if the focal distance of the former be to that of the latter as six to one, then the object would be magnified six times in diameter. But since the first image is magnified into a second image much larger, which is viewed through the eye lens, therefore the whole magnifying power is in a proportion compounded of de to ex, and of zx to zo. If the former proportion be as six to one, and the latter as eight to one, then the object will appear forty-eight (viz. six by eight) times larger in diameter through the telescope than to the naked eye.

The fourth species of reflecting telescope goes under the name of "Cassegrainian Telescope." It differs from the preceding, in having the small reflector convex, instead of concave; in consequence of which the small reflector must be placed nearer to the large reflector than the focus of the latter; then the rays from the large reflector fall convergingly upon the convex small reflector, and are by it sent back convergingly upon the lens, w x, &c. The chief difference be

H

tween this and the preceding telescope is, that in this the object appears inverted, because in it there is no image formed, or the rays do not cross each other, between the two reflectors. Also with the same magnifying power, &c. this telescope is shorter than the Gregorian, by twice the focal length of the small speculum.

To both those telescopes a long wire is fixed all along the outside of the tube, at the end of which there is a screw which works into an external projection, g, of the internal arm, H, and serves to move that arm with the small speculum nearer to or further from the large speculum, in order to adjust the focus of the instrument, according to the distance of the object. The action of this wire is easily understood; for it passes through a hole at F, where it is prevented going forwards or backwards by two shoulders, which are indicated by the figure: hence, when the observer looks through the hole, P, he turns with his hand the wire by the nut, Q, wich screws the projection, g, of the arm nearer or further, &c. until the object appears very distinct.

The largest reflecting telescope now existing was constructed by that excellent astronomer, Dr Herschel. It is a telescope, in which the observer looks through an eye lens down upon the large reflector, whose polished surface is forty-eight inches in diameter. Its focal length is about forty feet.

There are however two useful appendages to telescopes, which deserve to be briefly described. A finder, viz. a short telescope, A, fig. 3. is generally affixed to the tube of a large telescope, for the purpose of finding out an object expeditiously. This finder does not magnify the object more than four, six, or eight times; but it has a great field of view, so that through it a great part of the heavens may be seen at once. In the inside of its tube, and exactly at the focus of the eye glass, there are two slender wires, which cross each other in the axis of the telescope. Now the finder is adjusted by means of screws upon the tube of the great telescope in such manner, as that when an object, seen through the finder, appears to be near the crossing of the above-mentioned wires, it is at the same time visible through the great telescope: hence, when the observer wishes to view a small distant object, as a star, a planet, &c. he moves the instrument to one side or the other, until, by looking through the finder he brings the object nearly to coincide with the crossing of the wires;

and when that takes place, he immediately looks through the large telescope, &c.

A micrometer is an instrument, which is used with a telescope, for the purpose of measuring small angles. A great variety of micrometers have been contrived by various ingenious persons; and they are more or less complicated, more or less expensive, as also more or less accurate. See MICROMETER.

"Achromatic Telescope," is a name given to the refracting telescope invited by Mr. John Dollond, and so contrived as to remedy the aberration arising from colours, or the different refrangibility of the rays of light. The improvement made by Mr. Dollond in his telescopes, by making two object-glasses of crown glass, and one of flint, which was tried with success when concave eye-glasses were used, was completed by his son Peter Dollond; who, conceiving that the same method might be practised with success with convex eye-glasses, found, after a few trials, that it might be done. Accordingly he finished an object-glass of five feet focal length, with an aperture of 33 inches, composed of two convex lenses of crown-glass, and one concave of white flint-glass. But apprehending afterward that the apertures might be admitted still larger, he completed one of 3 feet focal length, with the same aperture of 3 inches. In the 17 inch improved achromatic telescope, the object-glass is composed of three glasses, viz. two convex of crown-glass, and one concave of white flint-glass; the focal distance of this combined objectglass is about seventeen inches, and the diameter of the aperture two inches. There are four eye-glasses contained in the tube, to be used for land objects; the magnifying power with these is near fifty times; and they are adjusted to different sights, and to different distances of the object, by turning a finger screw at the end of the outer tube. There is another tube, containing two eye-glasses that magnify about seventy times, for astronomical purposes. The telescope may be directed to any object by turning two screws in the stand on which it is fixed, the one giving a vertical motion, and the other a horizontal one. The stand may be inclosed in the inside of the brass tube.

The object-glass of the 23 and 3 feet telescopes is composed of two glasses, one convex, of crown-glass, and the other concave, of white flint-glass; and the diameters of their apertures are two inches and 24 inches. Each of them is furnished with two tubes; one for land objects,

containing four eye-glasses, and another with two eye-glasses for astronomical uses. They are adjusted by buttons on the outside of the wooden tube: and the vertical and horizontal motions are given by joints in the stands. The magnifying power of the least of these telescopes, with the eye-glass for land objects, is nearly fifty times, and with those for astronomical purposes, eighty times; and that of the greatest for land objects is nearly seventy times, but for astronomical observations eighty and a hundred and thirty times; for this has two tubes, either of which may be used as occasion requires. This telescope is also moved by a screw and rack-work, and the screw is turned by means of a hook's joint.

We must now say something of the specula of telescopes, having referred to this place from the article SPECULUM. The metals of reflecting telescopes are generally composed of thirty-two parts of copper and fifteen of grain tin, with the addition of two parts of arsenic, to render the composition more white and compact. It has been ascertained by a variety of experiments, that if one part of brass, and one of silver, be added to this composition, and only one of arsenic used, a most excellent metal will be obtained, which is the whitest, hardest, and most reflective.

The first composition is, however, for inexperienced persons, the best, as the easiest to cast, to grind, and polish. When this is employed, the copper and tin should be melted, and when mixed toge. ther should be poured into cold water, which will separate the mass into a num ber of small particles. These small pieces of metal are then to be collected and put into the crucible, along with the silver and brass after they have been melted together in a separate crucible, the proper quantity of arsenic is to be added, and a little powdered rosin thrown into the crucible before the metal is poured into the flasks. For the particular methods of grinding and polishing, we refer to Brewster's edition of Ferguson's Mechanics, vol. i.

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TELEPHIUM, in botany, a genus of the Pentandria Trigynia class and order. Natural order of Portulaceæ, Jussieu. Miscellaneæ, Linnæus. Essential character; calyx five-leaved; petals five, inserted into the receptacle; capsule onecelled, three-valved. There are two species, viz. T. imperati, true orpine; and T. oppositifolium, both natives of Barbary.

TELLER, an officer of the Exchequer,

in ancient records called tallier: there are four of these officers, whose duty is to receive all sums due to the king, and to give the clerk of the pells a bill to charge him therewith. They likewise pay all money due from the king, by warrant from the auditor of the receipt, and make weekly and yearly books, both of their receipts and payments, which they deliver to the lord-treasurer.

TELLINA, in natural history, a genus of the Vermes Testacea class and order: animal a tethys: shell bivalve, generally sloping on one side; in the fore part of one valve a convex, of the other, a concave fold; hinge with usually three teeth, the lateral ones smooth, in one shell. There are about eighty species, divided into sections. A. ovate and thickish. B. ovate and compressed. C. suborbicular. We shall notice one or two only. T. gari: shell oval, with transverse recurved striæ; lateral teeth obsolete; it inhabits the Indian ocean: the fore part is inflected and very rough, with transverse wrinkles, crossed in the middle by perpendicular striæ; sometimes cinereous, with brown rays; sometimes bluish, spotted with white, and white and red rays. T. cornea: shell globular, glabrous, horncolour, with a transverse groove. Mr. Pennant has described in the British Zoology: it inhabits the ponds and fresh water of Europe: it is not larger than a pea. The shell is pellucid, very finely striate across; within bluish white; without white, or pale or bluish-ash, with transverse black curves, one of which is more distinct; lateral teeth of the hinge elongated, hardly any middle ones.

This

TELL-TALE, in music, a moveable piece of ivory, or lead, suspended in the front of a chamber-organ, on one side of the keys, by a string; one side of the keys being attached to the bellows within, rises as they sink, and apprizes the performer in what degree the wind is exhausted.

TELLURIUM. See SYLVAN.

TEMPERAMENT, in music, the accommodation or adjustment of the imperfect sounds, by transferring a part of their defects to the more perfect ones, in order to remedy, in some degree, the false intervals of those instruments, the sounds of which are fixed; as the organ, harpsichord, piano-forte, &c.

TEMPERING of steel and iron, the rendering them either more compact and hard, or soft and pliant, according as the different uses for which they are wanted may require.

The hardest steel is the most brittle;

but in many cases it is necessary to diminish the hardness, and this operation is called tempering. The greatest difficulty consists in applying the proper degree of heat uniformly over the whole mass. The common method is, to judge by the colour assumed by the clean surface of steel when thus heated. The heat may be applied by the fire, or a pan of charcoal, or the fame of a candle or lamp, or by laying the piece upon sand to be gradually heated, or upon melted lead. Saw-makers, and those who manufacture springs, heat the article, rub it with grease, and then heat it further till the fumes take fire: this is called blazing, and affords a temper nearly the same as when the steel, by heat, has acquired a deep blue colour. When the temper is given from the colour, the first tinge is a faint straw colour: this is suitable to pen-knives and hard cutting tools. The next colour, which is purple, is rather too soft for a knife, and too brittle for a spring. After this follows the blue, of which there are several shades: the deepest is very soft, and this succeeded by a whitish-yellow, which indicates too great a degree of softness for any cutting tool. Mr. Hartley took out a patent for a method of tempering steel, which was done by heating the tools in oil raised to a high temperature. Pen-knives require a heat of 450° of Fahrenheit.

TEMPLARS, or TEMPLERS, a religious order instituted at Jerusalem, about the year 1118. Some religious gentlemen put themselves under the government of the patriarch of Jerusalem, renounced property, made the vow of celibacy and obedience, and lived like canons regular. King Baldwin assigned them an apartment in his palace. They had likewise lands given them by the king, the patriarch, and the nobility, for their main tenance. At first there were but nine of this order, and the two principal persons were, Hugo de Paganis, and Geoffrey of St. Omers. About nine years after their institution, a rule was drawn up for them, and a white habit assigned them, by Pope Honorius II. About twenty years after this, in the popedom of Eugenius III. they had red crosses sewed upon their cloaks, as a mark of distinction; and in a short time they were increased to about three hundred, in their convent at Jerusalem. They took the names of Knights Templars, because their first house stood near the temple dedicated to our Saviour, at Jerusalem. This order, after having performed many great exploits against

the infidels, became rich and powerful all over Europe; but the knights, abus. ing their wealth and credit, fell into great disorders and irregularities. Many crimes and enormities being alleged against them, they were prosecuted in France, Italy, and Spain; and at last, the pope, by his bull of the 22d of May, 1312, given in the council of Vienna, pronounced the extinction of the order of Templars, and united their estates to the order of St. John of Jerusalem.

TENACITY, a term applied to metals, by which is meant the power that a metallic wire, of a given diameter, has of resisting, without breaking, the action of a weight suspended from its extremity. The tenacity of different metals is very various: an iron wire, of one-tenth of an inch in diameter, will support, without breaking, about 5 cwt.; whereas one of lead will not support 30lb.

TENAILLE, in fortification, a kind of outwork, resembling a hornwork, but generally somewhat different; for, instead of two demi-bastions, it bears only in front a re-entering angle between the same wings, without flanks; and the sides are parallel. Tenaille, double or flank. ed, is a work whose front consists of four faces, making two re-entering angles, and three saliant; the wings or sides of this work being in like manner correspondent in the front of the gorge. Tenaille simple, a work having its front formed by two faces, which make a reentering angle, the sides running directly parallel from the head to the gorge. Tenaille of the place, is that which is comprehended between the points of two neighbouring bastions; that is to say, the curtain, the two flanks that are raised on the curtain, and the two sides of the bastions which face one another; so that it is the same with what is otherwise called the face of the fortress. Tenaille

of the fosse, is a low work, raised before the curtain in the middle of the fosse : it is of three sorts; the first is composed of a curtain, two flanks, and two faces; the rampart of the curtain, including the parapet and talus, is but five fathoms thick, but the rampart of the flanks and faces is seven. The second is composed only of two faces made on the lines of defence, whose rampart and faces are parallel. The third sort differs from the second only in this, that its rampart is parallel to the curtain of the place. All three sorts are good, and cannot be hurt by the besiegers' cannon, till they are masters of the covert way, and have planted their cannon there.