Sidebilder
PDF
ePub

we have called an ellipsis, will manifestly vary according to the law of the alternate motion in the mandrel. When the mandrel moves uniformly forward, the cut will be the common helix or screw; and the motion is used to make screws, though not very frequently, because good turners can easily make them by a notched cutting tool, called the screw.

The act of turning is so extensively applicable, that it would require a volume to describe its uses, and the methods of practising it. Every round thing which is made by human hands may be referred to this art, as one of its products. The largest columns, the most ponderous artillery, and the minutest pivots of watchwork, with all wheel-work, rotatory machines, vessels, &c. are worked in this method.

TURNSOLE. See LITMUS.

TURPENTINE. See RESINS. Turpentine, of which there are various kinds, are all products of some of the species of the pinus. From this genus are obtained not only turpentine, but rosin, pitch, tar, &c. which are employed so extensively in ship-building, and in the rigging also likewise in varnishes.

There are three varieties of pine turpentine, commonly known under that name in Europe: namely, 1. The common turpentine, obtained chiefly from the pinus sylvestris (Scotch fir). 2. The Strasburgh turpentine, yielded by the pinus picca (silver fir). And, 3. The Venice turpentine, procured from the pinus larix (larch). Of the three first mentioned turpentines, the Venice is the thinnest and most aromatic; the Strasburgh the next in these qualities; and the common is the firmest and coarsest. The two former are often adulterated by a mixture of the common turpentine and oil of turpentine; and it is to be observed, that the terms Venice and Strasburgh turpentine are not now appropriate, as they are procured from various countries.

Common turpentine is obtained largely in the pine forests in the south of France, in Switzerland, in the countries on the north of the Pyrenees, in Germany, and in many of the southern states of North America. The greater part of what is consumed in this country is imported from North America. The method of obtaining it is by making a series of incisions through the bark of the tree, from which the turpentine exudes, and falls down into holes, or other receptacles at the foot.

The process is described very accu

rately by Duhamel and others, as prac tised in the south of France. The fir is generally allowed to remain untouched till it is thirty or forty years old. When it is to be worked, which is early in the spring, a small hole is first made in the ground at the foot of the tree, the earth of which is well rammed, and serves as a receptacle for the juice. The coarse bark is then stripped off from the tree, a little above the hole, down to the smooth inner bark, after which a portion of the inner bark, together with a little of the wood, is cut out with a very sharp tool, so that there may be a wound in the tree about three inches square, and an inch deep. Immediately afterwards the turpentine begins to exude in very transparent drops, which escape chiefly from the wood immediately under the inner bark. The hotter the weather is, the greater is the supply of resin; and to facilitate the supply, the incisions are enlarged every three or four days, by cutting off thin slices, till at the end of the year it is about a foot and a half wide, and two or three inches deep. The whole time during which the turpentine flows is from the end of February to October. In the winter it entirely ceases, but in the ensuing spring a fresh incision is begun a little above the former, and managed in the same manner. This practice is continued annually for about twelve or fifteen years in some parts, and in others a shorter time, on the same side of the tree, till the later incisions are so high as to be out of reach without the assistance of steps; after which the contrary side of the tree is begun upon, and worked in a similar manner for as many years, during which time the first incisions are grown up, and are fit to be cut afresh. In this way, a healthy tree, in a favourable soil, may be made to yield from six to twelve, or more, pounds of turpentine annually, sometimes for a century; and even the timber is not soon injured by this constant drain. The flow of turpentine discontinues altogether about October, and the liquid resin collected during the year, from each tree, is put together for further purification. But a considerable quantity of the resin has concreted during that time around the incision, particularly as the heat declines; and in the winter, when it has hardened considerably, it is scraped off, and forms what is technically called barras, or in some provinces galipot, which differs from the more liquid turpentine in consistence, and probably contains a less proportion of essential oil. The galipot is

much used in making flambeaux, when mixed with suet; but the greater part of it, as well as the liquid turpentine, is subjected to further processes.

The Strasburgh turpentine, the produce of the silver fir, is the most fragrant of all the pine turpentines, and only inferior to the true Chio; but it is not often seen in the shops. It is obtained by rude incision of the bark by the peasants in the vast pine forests on the western Alps. The first cut is made as high as the hatchet will reach, and these are renewed annual. ly from above downwards to within a foot of the ground. But the finest kind of turpentine yielded by this tree is that which exudes from soft tubercles, or swellings of the inner bark. The peasants carry with them a large cow's horn, with the point of which they pierce these tubercles, and collect the juice in its hollow.

The true Venice turpentine, or resin of the larch, is obtained from the Tyrol and Savoy, and also from Dauphiny, by boring holes about an inch in diameter, with a gentle descent, in the most knotty parts of the tree. To these are adapted long perforated pegs, which serve as gutters to convey the juice into troughs placed beneath. It is yielded during the whole of the summer, and is simply purified by straining through hair sieves. A full grown larch will sometimes yield seven or eight pounds of turpentine annually for forty or fifty years.

TURQUOISE. The colour of this substance is pale sky-blue, passing into indigo-blue, and pale apple-green. It occurs in mass, or disseminated. Its fracture is even. Its hardness is nearly equal to that of glass; it is difficultly frangible. Specific gravity 3.12. Before the blowpipe its colour changes to greyish-white, and it becomes friable, but it does not melt. It is soluble in nitro-muriatic acid, and the European varieties are so in nitric acid; this menstruum, however, has no action on the Persian turquoises. It is composed, according to Buillon la Grange,

of

[blocks in formation]

Turquoise is generally considered as fossil-bone, or ivory penetrated by oxide of copper; it appears, however, from the above analysis, that the colouring matter is phosphate of iron. The oriental turquoises are found near Meched in Persia, also in Mount Caucasus, in Egypt and Arabia. The occidental ones are found in Languedoc in France, and in Hungary. Turquoise was formerly in some estimation for rings and other articles of personal ornament; but its value has greatly declined in modern times. The colour turquoise changes gradually by exposure to the air, from blue to green: when it arrives at this state, its commercial value is wholly extinct.

TURRITIS, in botany, tower-mustard, a genus of the Tetradynamia Siliquosa class and order. Natural order of Siliquosæ, Cruciformes, or Cruciferæ. Essential character: silique very long, angular; calyx converging, erect; corolla erect. There are eight species.

TURRÆA, in botany, a genus of the Decandria Monogynia class and order. Natural order of Trihilatæ. Meliæ, Jussieu. Essential character: calyx fivetoothed; petals five; nectary toothed, cylindrical, bearing the anthers at the mouth between the teeth; capsule pentacoccous; seeds two. There are five species.

TUSCAN order, in architecture, the first, simplest, and most massive of the five orders.

TUSSILAGO, in botany, colt's-foot, a genus of the Syngenesia Polygamia Superflua class and order. Natural order of Composite Discoidea. Corymbiferæ, Jussieu. Essential character: calyx scales equal, as long as the disk, somewhat membranaceous; down simple; recepta. cle naked. There are fourteen species.

TWILIGHT, that light, whether in the morning before sunrise, or in the evening after sunset, supposed to begin and end when the least stars that can be seen by the naked eye cease, or begin to appear. By means of the atmosphere it happens, that though none of the sun's direct rays can come to us after it is set, yet we still enjoy its reflected light for some time, and night approaches by degrees; for after the sun is hidden from our eyes, the upper part of our atmosphere remains for some time exposed to its rays, and from thence the whole is illuminated by reflection. But as the sun grows lower and lower, that portion of the atmosphere which is above our horizon becomes enlightened till the sun has

got eighteen degrees below it; after which it ceases to be illuminated thereby, till it has got within as many degrees of the eastern side of the horizon; at which time it begins to illuminate the atmosphere again, and in appearance to diffuse its light throughout the heavens, which continues to increase till the sun be up. Hence it is, that during that part of the year in which the sun is never eighteen degrees below our horizon, there is a continued twilight from sunsetting to sun-rising. Now that part of the year in the latitude of London is, while the sun is passing from about the fifth degree of Gemini to the twelfth of Cancer; that is, from the middle of May to the middle of July.

For

As the twilight depends on the quantity of matter in the atmosphere fit to reflect the sun's rays, and also on the height of it, (for the higher the atmosphere is, the longer will it be before the upper parts of it will cease to be illuminated,) the duration of it will be various. instance, in winter, when the air is condensed with cold, and the atmosphere upon that account lower, the twilight will be shorter; and in summer, when the limits of the atmosphere are extended by the rarefaction and dilatation of the air of which it consists, the duration of the twilight will be greater. And for the like reason, the morning twilight, the air being at that time condensed and contracted by the cold of the preceding night, will be shorter than the evening one, when the air is more dilated and expanded.

The beginning and end of twilight has been variously stated, by different observers; but, in our latitude, it may be said to begin and end when the sun is about eighteen degrees below the horizon: hence, when refraction is allowed for, the atmosphere must be capable of reflecting sensible light at the height of about forty miles. The duration of twilight is greater or less as the sun moves more or less obliquely with respect to the horizon: hence it is shortest near the time of the equinoxes, because the equinoxial intersects the horizon less obliquely than any lesser circle parallel to it. Dr. Long has calculated the duration of twilight in different latitudes, and for the several different declinations of the sun: the result he laid before the public in the following table, where the letters c d signify that it is then continual day; cn con

tinual night; and wn, that the twilight lasts the whole night:

૧૦

[blocks in formation]
[merged small][ocr errors]

1323416

Enters h. m h. m.

h. m h. m. h. m.

1 18 1 21 1 28 1 41 2 8

2 39 w

1 16 1 19

1 25

1 36

1 58

2 19

1 13 1 15

1 20

1 28

1 43

1 55

1 12 1 13

1 17

1 24

1 35

1 44

1 55 2 2 2 10 2 33 3 8 4 18 w 2 12 2 25 2 41 3 55 w nw nw 3 3 w nw nw nw nc dc dc dcd h. m h. m. h. m. h. m. h. m. h. m h. m. h. m. h. m h.m.h.m nw nc dc dc dcd

[blocks in formation]
[blocks in formation]

32222

08

532

8 2 27 2 56 8 41 5

2 57 4 8 2 18 2 43 3 26 11 38 11 1410 32 8 38 c

410 24 9 30

7 46

==

[ocr errors]
[ocr errors]

1225 P

[ocr errors]

==

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

TYCHONIC system, or hypothesis, an or

der or arrangement of the heavenly bodies, of an intermediate nature between the Copernican and Ptolemaic, or parti

cipating alike of them both. This system had its name and original from Tycho Brahe, a nobleman of Denmark, who lived in the latter part of the last century. This philosopher, though he approved of the Copernican system, yet could not reconcile himself to the motion of the earth, and being, on the other hand, convinced the Ptolemaic scheme could not be true, he contrived one different from either. In this the earth has no motion allowed it, but the annual and diurnal phenomena are solved by the motion of the sun about the earth, as in the Ptolemaic scheme; and those of Mercury and Venus are solved by this contrivance, though not in the same manner, nor so simply and naturally, as in the Copernican system. The Tychonic system then supposed the earth in the centre of the world, that is, of the firmament of stars, and also of the orbits of the sun and moon; but at the same time it made the sun the centre of the planetary motions, viz. of the orbits of Mercury, Venus, Mars, Jupiter and Saturn. Thus the sun, with all its planets, was made to revolve about the earth once a year, to solve the phenomena arising from the annual motion; and every twenty-four hours to account for those of the diurnal motion.

TYGER. See FELIS.

TYLE, or TILE, in building, a sort of thin, fictitious, laminated brick, used on the roofs of houses; or more properly, a kind of fat clayey earth, kneaded and moulded, of a just thickness, dried and burnt in a kiln like a brick, and used in the covering and paving of houses. See BRICK.

There are various kinds of tyles, for the various occasions of building; as plain, thack, ridge, roof, crease, gutter, pan, crooked, Flemish, corner, hip, dormar, scallop, astragal, traverse, paving, and Dutch tyles.

Flemish or Dutch tyles are of two kinds, ancient and modern. The ancient were used for chimney foot-paces; they were painted with antique figures, and frequently with postures of soldiers; some with compartments, and sometimes with moresque devices: but they come much short of the design and colours of the modern ones. The modern Flemish tyles are commonly used plastered up in the jambs of chimneys, instead of chimney-corner stones. These are better glazed, and such as are painted (for some are only white) are done with more curious figures, and more lively co

lours, than the ancient ones. But both kinds seem to be made of the same whitish clay as our white glazed earthen ware; the modern ones are commonly painted with birds, flowers, &c. The ancient ones are only five inches and a quarter square, and about three-quarters of an inch thick; the modern ones six inches and a half square, and three-quarters of an inch thick.

TYMPAN, or TYMPANUM, in architecture, the area of a pediment, being that part which is on a level with the naked of the frieze. Or it is the space included between the three cornishes of a triangular pediment, or the two cornishes of a circular one. Sometimes the tympan is cut out, and the part filled with an iron lattice, to give light, and sometimes it is enriched with sculpture in basso relievo.

TYMPAN, among printers, a double frame belonging to the press, covered with parchment, on which the blank sheets are laid in order to be printed off. See PRINTING.

TYMPANUM, or TYMPAN, in mechanics, a kind of wheel placed round an axis, or cylindrical beam, on the top of which are two levers or fixed staves, for the more easy turning the axis, in order to raise a weight required. The tympanum is much the same with the peritrochium, but that the cylinder of the axis of the peritrochium is much shorter and less than the cylinder of the tympanum.

TYMPANUM of a machine, is also used for a hollow wheel, wherein one or more people, or other animals, walk, to turn it; such as that of some cranes, calenders, &c.

TYPE, a copy, image, or resemblance of some model. This word is much used among divines, to signify a symbol, sign, or figure of something to come; in which sense it is commonly used with relation to antitype, which is the thing itself, whereof the other is a type or figure.

TYPHA, in botany, a genus of the Monoecia Triandria class and order. Natural order of Calamariæ. Typhæ, Jussieu. Essential character: male, ament cylindrical; calyx indistinct, three-leaved; corolla none; female, ament cylindrical, below the males; calyx a villose hair; corolla none; seed one, placed on a capillary down. There are two species, viz. T. latifolia, great cat's tail, or reed mace ; and T. angustifolia, narrow-leaved cat's tail.

TYPOGRAPHY, the art of printing. See PRINTING and STEREOTYPE.

U.

U, vowel of our alphabet, is formed

Or u, the twentieth letter, and fifth

:

in the voice by a round configuration of the lips, and a greater extrusion of the under one than in forming the letter o, and the tongue is also more cannulated. The sound is short in crust, must, tun, tub but is lengthened by a final e, as in tune, tube, &c. In some words it is rather acute than long; as in brute, flute, lute, &c. It is mostly long in polysyllables; as in union, curious, &c. but in some words it is obscure, as in nature, venture, &c. This letter, in the form, V, or v, is properly a consonant, and as such is placed before all the vowels; as in vacant, venal, vibrate, &c. Though the letter v and u had always two sounds, they had only the form v, till the beginning of the fourth century, when the other form was introduced, the inconvenience of expressing two different sounds by the same letter having been observed long before. In numerals V stands for five, and with a

dash added at top, thus, V, it signifies

5000.

VACCINATION. See SURGERY.

VACCINIUM, in botany, bilberry, or whortleberry, a genus of the Octandria Monogynia class and order. Natural order of Bicornes. Erica, Jussieu. Essential character: calyx superior; corolla onepetalled; filaments inserted into the receptacle; berry four-celled, many-seeded. There are twenty-seven species.

VACUUM, in philosophy, denotes a space empty, or devoid, of all matter or body. It has been the opinion of some philosophers, particularly the Cartesians, that nature admits not a vacuum, but that the universe is entirely full of matter: in consequence of which opinion they were obliged to assert, that if every thing contained in a vessel could be taken out or annihilated, the sides of the vessel, however strong, would come together; but this is contrary to experience, for the greatest part of the air may be drawn out of a vessel by means of the air-pump, notwithstanding which it will remain whole, if its sides are strong enough to support the weight of the incumbent atmosphere. Should it be objected here, that it is impossible to extract all the air out of a vessel, and that there will not be a vacuum on that account; the answer is, that since a very great part of the air that was in the vessel may be drawn out, as appears VOL. XII,

by the more quick descent of light bodies in a receiver when exhausted of its air, there must be some vacuities between the parts of the remaining air; which is sufficient to constitute a vacuum. Indeed, to this it may be objected by a Cartesian, that those vacuities are filled with materia subtilis, that passes freely through the sides of the vessel, and gives no resistance to the falling bodies: but as the existence of this materia subtilis can never be proved, we are not obliged to allow the objection, especially since Sir Isaac Newton has found that all matter affords a resistance nearly in proportion to its density. There are many other arguments to prove this, particularly the motions of the comets through the heavenly regions, without any sensible resistance, the different weight of bodies of the same bulk, &c. All the parts of spaces, says Sir Isaac Newton, are not equally full; for if they were, the specific gravity of the fluid which would fill the region of the air, great density of its matter, give way to could not, by reason of the exceeding the specific gravity of quicksilver, gold, or any body, how dense soever; whence neither gold, nor any other body, could descend in the air; for no body can descend in a fluid, unless it be specifically heavier than it. But if a quantity of matter may, by rarefaction, be diminished in a given space, why may it not diminish in infinitum? And if all the solid particles of bodies are of the same density, and cannot be rarefied, without leaving pores, there must be a vacuum.

VADE mecum, or VENI mecum, a Latin phrase, used in English to express a thing that is very handy and familiar, and which one usually carries about with them; chiefly applied to some favourite book.

VAGINA, properly signifies a sheath, or scabbard: and the term vagina is used, in architecture, for the part of a terminus, because resembling a sheath, out of which the statue seems to issue.

VAGINALIS, the sheath-bill, in natural history, a genus of birds of the order Gralla. Generic character: bill strong, thick, compressed; upper mandible covered above with a moveable horny sheath; nostrils placed before the sheath; face naked and papillous; wings with an obtuse excrescence under the flexure; claws grooved. V. alba, or the white sheath-bill, the only species known,

T

« ForrigeFortsett »