land resumed it, by creating eight knights: however, the revolution unsettled it again, and it lay neglected till queen Anne, in 1703, restored it to the primitive design, of twelve knights of St. Andrew. King George I. in the first of his reign, confirmed the statutes signed by queen Anne, with the addition of several more, among which was that of adding rays of glory to surround the figure of St. Andrew, which hangs at the collar and though from the reforma. tion to George I. both elections and instalments had been dispensed with, his majesty ordered that chapters of election should, for the future, be held in the royal presence; to which end he ordered the great wardrobe to provide the knights brethren, and officers, with such mantles as the statutes of the said order appointed.

THLASPI, in botany, bastard-cress, a genus of the Tetradynamia Siliculosa class and order. Natural order of Siliquosæ or Cruciformes. Cruciferæ, Jussieu. Essen. tial character: silicle emarginate, obcor. date, many-seeded; valves boat-shaped, margined and keeled. There are fourteen species.

THOA, in botany, a genus of the Monoecia Polyandria class and order. Natural order of Urticæ, Jussieu. Essential character: calyx and corolla none; male, stamens numerous, at the joints of the spike; female, germs two, at the base of the male spike, one on each side, sessile; stigma three or four cleft; seed in a brittle shell, covered with a bristly web. There is only one species, riz. T. urens.

THOLES, in marine affairs, small pins driven perpendicularly into the gunwale of a boat, and serving to retain the oars in that space which is called the row-lock; sometimes there is only one pin to each oar, as in boats navigated in the Mediterranean Sea in that case the oar is retained upon the pin, by means of a strop, or of a cleat, with a hole through it, nailed on the side of the oar.

THOUINIA, in botany, so named in honour of Mons. André Thouin, fellow of the National Institute, and professor of Horticulture in the French Museum, a genus of the Pentandria Monogynia class and order. Natural order of Convolvuli, Jussieu. Essential character: corolla one-petalled, bell-shaped, inferior, hispid on the outside; style simple; drape. There is but one species, viz. T. spectabilis, a native of Madagascar, where it was found by Com

merson.

THONSCHIEFER, in mineralogy, slate, is divided into three sub-species: 1. The common argillaceous schistus, which is composed of silex, alumina, oxide of iron, and proportions of carbonated lime and magnesia: it is used for covering houses, and the strait-foliated bluish-grey varieties are employed as writing slates: the softer and more compact varieties are made into slate pencils. See SCHISTUS, also SLATE. 2. Hone slate, called by Kirwan novaculite: its colour is a greenish-grey, or smokegrey, passing to olive and mountain-green: It occurs in mass, and has a glimmering lustre : its fracture in the great is slaty; in the small, splintery: its fragments are tabular. It is more or less translucent on the edges: it is moderately hard, and not very frangible. Specific gravity 2.7. It does not effervesce with acids, neither is it fusible by the blow-pipe without addition. It is cut into hones for sharpening the finer kinds of steel instruments. It is found in Bareith, Seifendorf in Saxony, in Bohemia, and the Levant; hence it is called Turkish hone: also in some parts of North Wales. 3. Black chalk: its colour is greyish or bluish black. It occurs in mass: the fragments are tabular or splintery. It stains the fingers, and gives a somewhat glossygrey streak. It is meagre, but smooth to the touch; is soft, and very easily frangible. Before the blow-pipe, without addition, it acquires a thin varnish, but does not melt. It has been analysed, and found to consist of

THRASHING, or THRESHING, in agriculture, the art of beating the corn out of the ears. There are several ways of separat ing corn from the ear; the first by beating it with a flail, which is properly what is called thrashing. The other method, still practised in several countries, is to make mules, or horses, trample on it, backwards and forwards; this is properly what the ancients

called-tritura and trituratio. The Hebrews

used oxen therein, and sometimes yoked four together for this purpose. Another way among the ancients was with a kind of sledge, made of boards joined together, and loaden with stones or iron, upon which a man was mounted, and the whole drawn over the corn by horses: this instrument was called traha or tribula. It is a rule among husbandmen, that the season for thrashing is as soon as the corn has sweated in the heap or mow. Thrashing machines are now much in use with the farmers on a large scale.

THREAD, a small line, made up of a number of fine fibres of any vegetable or animal substance, such as flax, cotton, or silk thread.

THREATENING letter. If any person shall send any letter threatening to accase any other person of a crime punishable with death, transportation, pillory, or other infamous punishment, with a view to extort money from him, he shall be punished at the discretion of the court, with fine, imprisonment, pillory, whipping, or transportation: 80. G. II. c. 24. And if the writer of a threatening letter deliver it himself, and do not send it, he is guilty of felony under this act.

THRINAX, in botany, a genus of the Appendix Palmæ class and order. Natural order of Palms. Essential character: calyx six-toothed; corolla none; stigma funnelform, oblique; berry one-seeded. There is only one species, yiz. T. parviflora, palmeto royal, or palmeto-thatch, a native of Jamaica and Hispaniola.

THRUSH. See TURDUS.

THRYALLIS, in botany, a genus of the Decandria Monogynia class and order. Natural order of Tricoccæ. Acera, Jussien. Essential character: calyx five-parted; pe tals five; capsule tricoccous. There is but one species, viz. T. brasiliensis, a little shrub, with round jointed branches; small yellow flowers; fruits tricoccous, or threegrained; it is a native of Brasil.

THUJA, in botany, arbor vitæ, a genus of the Monoecia Monadelphia class and or der. Natural order of Coniferæ. Essential character: male, calyx scale of an ament; corolla none; stamina four; female, calyx of a strobile, with a two-flowered scale; corolla none; pistil one; nut one, girt with a membranaceous wing. There are four species. We shall notice the T. occidentalis, American, or common, arbor vitæ ; this tree has a strong woody trunk, rising to the height of forty feet; the bark, while young, is smooth, and of a dark brown colour, but as the trees advance, the bark becomes cracked, and less smooth, the branches are produced irregularly on every side, standing almost horizontal, the young slender shoots frequently hang down; the young branches are flat, and the small leaves are placed over each other like the scales of fish; the flowers are produced from the side of the young branches, very near to the foot stalk; the males grow in oblong catkins, between these the females are collected in form of cones; when the former have shed their farina, they soon drop off, the latter are succeeded by oblong cones, or strobiles, having obtuse smooth scales, containing one or two oblong seeds.

THUMERSTONE, in mineralogy, a species of the flint genus: common colour is brown of various degrees of intensity: it is seldom found massive, often disseminated, but most frequently crystallized. Specific gravity about 3.3. It melts easily before the blow-pipe, without addition, into a greenish, white, semi-transparent glass. The constituent parts are,

the continent, also in our country in Corn. wall.

clouds are formed in the neighbourhood, even at these short intervals. Hence it appears, that during this particular period there must be some natural cause operating for the production of this phenomenon, which does not take place at other times. This caunot be the niere heat of the wea

The explosion, if high in the air, and remote from us, will do no mischief; but when near, it may, aud has in a thousand instances, destroyed trees, animals, &c. This proximity, or small distance, may be estimated nearly by the interval of time between seeing the flash of lightning, and hearing the report of the thunder, estimat ing the distance after the rate of 1,142 feet per second of time, or 33 seconds to the mile. Dr. Wallis observes, that commonly the difference between the two is about seven seconds, which, at the rate above mentioned, gives the distance almost two miles. But sometimes it comes in a second or two, which argues the explosion very near us, and even among us. And in such cases, the doctor assures us, he has sometimes foretold the mischiefs that happened.

Although in this country thunder may happen at any time of the year, yet the months of July and August are those in which it may almost certainly be expected. Its duration is of very uncertain continu ance; sometimes only a few peals will be heard at any particular place during the whole season; at other times the storm will return at the interval of three or four days, for a month, six weeks, or even longer; not that we have violent thunder in this country directly vertical in any one place so frequently in any year, but in many seasons it will be perceptible that thunder

weather without any thumder; and besides, though not common, thunder is sometimes heard in the winter also. As therefore the heat of the weather is common to the whole summer, whether there be thunder or not, we must look for the causes of it in those

phenomena, whatever they are, which are peculiar to the months of July, August, and the beginning of September. Now it is generally observed, that from the month of April, an east or south-east wind genenerally takes place, and continues with litthe interruption till towards the end of June. At that time, sometimes sooner and sometimes later, a westerly wind takes place ; but as the causes producing the east wind are not removed, the latter opposes the west wind with its whole force. At the place of meeting, there is naturally a most vehement pressure of the atmosphere, and friction of its parts against one another; a calm ensues, and the vapours brought by both winds begin to collect and form dark clouds, which can have little motion either way, because they are pressed almost equally on all sides. For the most part, however, the west wind prevails, and what little motion the clouds have is towards the east: whence the common remark in this country, that "thunder-clouds move against the wind." But this is by no means universally true for if the west wind happens to be excited by any temporary cause before its natural period when it should take place, the east wind will very frequently get the better of it; and the clouds, even although thunder is produced, will move westward. Yet in either case the motion is so slow, that the most superficial observers cannot help taking notice of a considerable resistance in the atmosphere.

When lightning acts with extraordinary violence, and breaks or shatters any thing, it is called a thunderbolt, which the vulgar, to fit it for such effects, suppose to be a hard body, and even a stone. But that we need not to have recourse to a hard solid body to account for the effects commonly attributed to the thunderbolt, will be evident to any one, who considers those of gunpowder, and the several chemical ful

minating powders, but more especially the astonishing powers of electricity, when only collected and employed by human art, and much more when directed and exercised in the course of nature.

When we consider the known effects of electrical explosions, and those produced by lightning, we shall be at no loss to account for the extraordinary operations vulgarly ascribed to thunderbolts. As stones and bricks struck by lightning are often found in a vitrified state, we may reasonably suppose, with Beccaria, that some stones in the earth, having been struck in this manner, gave occasion to the vulgar opinion of the thunderbolt.

Thunder-clouds are those clouds which are in a state fit for producing lightning and thunder. From Beccaria's exact and circumstantial account of the external appearances of thunder-clouds, the following particulars are extracted. The first appearance of a thunder storm, which usually hap. pens when there is little or no wind, is one dense cloud, or more, increasing very fast in size, and rising into the higher regions of the air. The lower surface is black and nearly level; but the upper finely arched, and well defined. Many of these clouds often seem piled upon one another, all arched in the same manner; but they are continually uniting, swelling, and extending their arches. At the time of the rising of this cloud, the atmosphere is commonly full of a great many separate clouds, that are motionless, and of odd whimsical shapes. All these, upon the appearance of the thunder-cloud, draw towards it, and become more uniform in their shapes as they approach; till, coming very near the thundercloud, their limbs mutually stretch towards one another, and they immediately coalesce into one uniform mass. These he calls adscititious clouds, from their coming in to enlarge the size of the thunder-cloud. But sometimes the thunder-cloud will swell, and increase very fast, without the conjuction of any adscititious clouds; the vapours in the atmosphere forming themselves into clouds wherever it passes. Some of the ad. scititious clouds appear like white fringes, at the skirts of the thunder-cloud, or under the body of it, but they keep continually growing darker and darker, as they approach to unite with it. When the thunder-cloud is grown to a great size, its lower surface is often ragged, particular parts being detached towards the earth, but still connected with the rest. Sometimes the

lower surface swells into various large protuberances bending uniformly downward; and sometimes one whole side of the cloud will have an inclination to the earth, and the extremity of it nearly touch the ground. When the eye is under the thunder-cloud, after it is grown larger, and well formed, it is seen to sink lower, and to darken prodigiously; at the same time that a number of small adscititious clouds (the origin of which can never be perceived) are seen in a rapid motion, driving about in very uncertain directions under it. While these clouds are agitated with the most rapid motions, the rain commonly falls in the greatest plenty, and if the agitation be exceedingly great, it commonly hails. While the thunder-cloud is swelling, and extending its branches over a large tract of country, the lightning is seen to dart from one part of it to another, and often to illuminate its whole mass. When the cloud has acquired a sufficient extent, the lightning strikes between the cloud and the earth, in two opposite places, the path of the lightning lying through the whole body of the cloud and its branches. The longer this lightning continues, the less dense does the cloud become, and the less dark its appearance; till at length it breaks in different places, and shows a clear sky. These tbunder-clouds were sometimes in a positive as well as a negative state of electricity. The electricity continued longer of the same kind, in proportion as the thunder-cloud was simple and uniform in its direction: but when the lightning changed its place, there commonly happened a change in the electricity of the apparatus over which the clouds passed. It would change suddenly after a very violent flash of lightning, but the change would be gradual when the lightning was moderate, and the progress of the thunder-cloud slow. See Priestley's History of Electricity.

THYMBRA, in botany, a genus of the Didynamia Gymnospermia class and order. Natural order of Verticillata or Labiatæ. Essential character: calyx sub-cylindrical, two-lipped, scored on each side with a villose line; style semibifid. There are three species.

THYMUS, in botany, thyme, a genus of the Didynamia Gymnospermia class and order. Natural order of Verticillata or Labiatæ. Essential character: throat of the two-lipped calyx closed with villose hairs. There are twenty-two species.

THYNNUS, in natural history, a genus

of insects of the order Hymenoptera: mouth horny, with an incurved mandible, the jaw short and straight; lip longer than the jaw, membranaceous at the tip, and trifid, the middle division emarginate; tongue very short, involute; four feelers, equal, filiform; antennæ cylindrical, the first joint thicker. There are four species: three of New Holland, and one of Africa. Specimens of them all are to be found in Sir Joseph Banks's

museum.

TIARELLA, in botany, a genus of the Decandria Digynia class and order. Natural order of Succulenta. Saxifragæ, Jussieu. Essential character: calyx five-parted; corolla five-petalled, inserted into the calyx; petals entire; capsule one-celled, two-valved, with one valve larger. There are two species; viz. T. cordifolia, heart-leaved tiarella; and T. trifoliata, three-leaved tiarella, both natives of the northern parts of America and Asia.

TIDES, two periodical motions of the waters of the sea, called the flux and reflux, or the flow and ebb. The cause of the tides is the attraction of the sun and moon, but chiefly of the latter; the waters of the immense ocean, forgetful, as it were, of their natural quietus, move and roll in tides, obse. quious to the strong attractive power of the moon, and weaker influence of the sun. See ASTRONOMY.

That the tides may have their full motion, the ocean in which they are produced ought to be extended from east to west 90', or a quarter of a great circle of the earth, at least; because the places where the moon raises most, and most depresses the water, are at that distance from one another. Hence it appears, that it is only in the great oceans that such tides can be produced; and why, in the large Pacific ocean, they exceed those in the Atlantic ocean; hence also it is obvious, why the tides are not so great in the torrid zone, between Africa and America, where the ocean is narrower, as in the temperate zones on either side; and from this also, we may understand why the tides are so small in islands that are very far distant from the shores. It is manifest, that, in the Atlantic ocean, the water cannot rise on one shore but by descending on the other; so that, at the intermediate dis tant islands, it must continue at about a mean height between its elevation on the one and on the other shore. As the tides pass over shoals, and run through streights into bays of the sea, their motion becomes more various, and their height depends on a great VOL. VI.

many circumstances. The tide that is pro duced on the western coast of Europe corresponds to the theory above described: thus, it is high water on the coast of Spain, Portugal, and the West of Ireland, about the third hour after the moon has passed the meridian: from thence it flows into the adjacent channels, as it finds the easiest passage. One current from it, for example, rans up by the south of England, and another comes in by the north of Scotland: they take a considerable time to move all this way, and it is high water sooner in the places to which they first come; and it begins to fall at those places, while the two currents are yet going on to others that are further in their course. As they return, they are not able to raise a tide; because the water runs faster off than it returns, til by a new tide propagated from the ocean, the return of the current is stopped, and the water begins to rise again. The tide takes twelve hours to come from the ocean to London bridge, so that, when it is high water there, a new tide is already come to its height in the ocean; and, in some intermediate place, it must be low water at the same time. In channels, therefore, and narrow seas, the progress of the tides may be, in some respects, compared to the motion of the waves of the sea. It may be observed, that when the tide runs over shoals, and flows upon flat shores, the water is raised to a greater height than in the open and deep oceans that have steep banks; because the force of its motion cannot be broken, upon these level shores, till the water rises to a greater height. If a place communicates with two oceans (or two different ways with the same ocean, one of which is a readier and easier passage) two tides may arrive at that place in different times, which, interfering with each other, may produce a greater variety of phenomena.

An extraordinary instance of this kind is mentioned at Bathsha, a port in the kingdom of Tonquin in the East Indies, of northern latitude 20° 50'. The day in which the moon passes the equator, the water stagnates there without any motion: as the moon removes from the equator, the water begins to rise and fall once a day; and it is high water at the setting of the moon, and low water at her rising. This daily tide increases for about seven or eight days, and then decreases for as many days by the same degrees, till this motion ceases when the moon has returned to the equator. When she has passed the equator, and declines to

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