and consists of three petals; and its fruit is an oblong unilocular pod, contracted in the middle, and containing two oblong, obtuse, and gibbous seeds. There is but one species, found in the Indies, a tree, stem herbaceous, hairy, procumbent. The branches trail on the ground, and the germ, after flowering, thrusts itself under grond, where the food is formed and ripened.

ARACHNOIDES, in zoology, a name given to those echini marini, or sea-hedgehogs, which are of a circular form, but variously indented at the edges. See ECHINUS.

ARALIA, berry-bearing angelica, in botany, a genus of the Pentandria Pentagynia class of plants, the flowers of which are collected into an umbel, of a globose figure, with a very small involucrum ; the perianthium is very small, divided into five parts, and placed on the germen; the corolla consists of five, ovato-acute, sessile, reflex petals; the fruit is a roundish, coronated, striated berry; having five cells; the seeds are single, hard, and oblong. There are four divisions, viz. A. leaves entire; B. leaves lobed; C. leaves in finger-like divisions; D. leaves decompound, and more than decompound. In the first there are three species; in the second one; in the third two; and in the fourth four.

ARANEA, in natural history, the spider, a genus of insects of the order Aptera. Gen. char. legs eight; eyes eight, sometimes six; mouth furnished with two hooks, or holders; feelers two, jointed, the tips of which in the male distinguish the sex; abdomen terminated by papillæ, or teats, through which the insect draws the thread.

One of the largest of the European spiders is the Aranea diadema of Linnæus, which is extremely common in our own country, and is chiefly seen during the autumnal season in gardens, &c. The body of this species, when full grown, is not much inferior in size to a small hazel nut: the abdomen is beautifully marked by a longitudinal series of round, or drop-shaped milkwhite spots, crossed by others of similar appearance, so as to represent, in some degree, the pattern of a small diadem. This spider, in the months of September and October, forms, in some convenient spot or shelter, a large round close, or thick web of yellow silk, in which it deposits its eggs, guarding the round web with a secondary one of a looser texture. The young are hatched in the ensuing May, the parent insects dying towards the close of autumn.'

The Aranea diadema being one of the largest of the common spiders, serves to exemplify some of the principal characters of the genus in a clearer manner than most others. At the tip of the abdomen are placed five papillæ or teats, through which the insect draws its thread; and as each of these papillæ is furnished with a vast number of foramina or outlets, disposed over its whole surface, it follows that what we commonly term a spider's thread, is in reality formed of a collection of a great many distinct ones; the animal possessing the power of drawing out more or fewer at pleasure; and if it should draw from all the foramina at once, the thread might consist of many hundred distinct filaments. The eyes, which are situated on the upper part or front of the thorax, are eight in number, placed at a small distance from each other, and having the appearance of the stemmata in the generality of insects. The fangs or piercers, with which the animal wounds its prey, are strong, curved, sharp-pointed, and each furnished on the inside, near the tip, with a small oblong hole or slit, through which is evacuated a poisonous fluid into the wound made by the point itself, these organs operat ing in miniature on the same principle with the fangs in poisonous serpents. The feet are of a highly curious structure; the two claws with which each is terminated being furnished on its under side with several parallel processes resembling the teeth of a comb, and enabling the animal to dispose and manage with the utmost facility the disposition of the threads in its web, &c.

Aranea tarantula, or Tarantula spider, of which so many idle recitals have been detailed in the works of the learned, and which, even to this day, continues in some countries to exercise the faith and ignorance of the vulgar, is a native of the warmer parts of Italy and other warm European regions, and is generally found in dry and sunny plains. It is the largest of all the European spiders, but the extraordináry symptoms supposed to ensue from the bite of this insect, as well as their supposed cure by the power of music alone, are entirely fabulous, and are now sufficiently exploded among all rational philosophers. The gigantic Aranea avicularia, or Bird-catching spider, is not uncommon in many parts of the East Indies and South America, where it resides among trees; frequently seizing on small birds, which it destroys by wounding with its fangs, and afterwards sucking their blood: during the early part of the

last century a project was entertained by a French gentleman, Monsieur Bon of Montpelier, of instituting a manufacture of spiders' silk, and the Royal Academy, to which the scheme was proposed, appointed the ingenious Reaumur to repeat the experiments of Monsieur Bon, in order to ascertain how far the proposed plan might be carried; but, after making the proper trials, Mr. Reaumur found it to be impracticable, on account of the natural disposition of these animals, which is such as will by no means admit of their living peaceably together in large numbers. Mr. Reaumur also computed that 663522 spiders would scarcely furnish a single pound of silk. Monsieur Bon, however, the first projector, carried his experiments so far as to obtain two or three pair of stockings and gloves of this silk; which were of an elegant grey colour, and were presented, as samples, to the Royal Academy. It must be observed that in this manufacture it is the silk of the eggbags alone that can be used, being far stronger than that of the webs. Monsieur Bon collected twelve or thirteen ounces of these, and having caused them to be well cleared of dust, by properly beating with sticks, he washed them perfectly clean in warm water. After this they were laid to steep, in a large vessel, with soap, saltpetre, and gum arabic. The whole was left to boil over a gentle fire for three hours, and were afterwards again washed to get out the soap; then laid to dry for some days, after which they were carded, but with much smaller cards than ordinary. The silk is easily spun into a fine and strong thread: the difficulty being only to collect the silk-bags in sufficient quantity. There remains one more particularity in the history of spiders, viz. the power of flight. It is principally in the autumnal season that these diminutive adventurers ascend the air, and contribute to fill it with that infinity of floating cobwebs which are so peculiarly conspicuons at that period of the year. When inclined to make these aerial excursions, the spider ascends some slight eminence, as the top of a wall, or the branch of a tree; and turning itself with its head towards the wind, ejaculates several threads, and rising from its station, commits itself to the gale, and is thus carried far beyond the height of the loftiest towers, and enjoys the pleasure of a clearer atmosphere. During their flight it is probable that spiders employ themselves in catching such minute winged insects as may happen to occur in their pro

gress; and when satisfied with their journey and their prey, they suffer themselves to fall, by contracting their limbs, and gradually disengaging themselves from the thread which supports them. See Plate I. Entomology, fig. 7 and 8.

ARAUCARIA, in botany, a genus of the Dioecia Monadelphia class and order. Male, calyx scales of an ament, terminated by a leafet; no corol; antheræ 10 to 12, without filaments. Female, calyx an ament with many germs; no corol; stigma twovalved, unequal; seeds numerous, in a roundish cone.

ARBITER, in civil law, a judge nominated by the magistrate, or chosen voluntarily by two parties, in order to decide their differences according to law.

The civilians make this difference between arbiter and arbitrator; though both ground their power on the compromise of the parties, yet their liberty is different, for an arbiter is to judge according to the usages of the law, but the arbitrator is permitted to use his own discretion, and accommodate the difference in the manner that appears to him most just and equitable.

ARBITRATION, a power given by two or more contending parties to some person or persons to determine the dispute between them: if the two do not agree it is usual to add that another person be called as umpire, to whose sole judgment it is then referred. The submission to arbitration is the authority given by the parties in controversy to the arbitrators, to determine and end their grievances; and this being a contract or agreement, must not be strictly taken, but largely, according to the intent of the parties submitted. There are five things incident to an arbitration: 1. Matter of controversy. 2. Submission. 3. Parties to the submission. 4. Arbitrators. 5. Giving up the arbitration. Matters relating to a freehold, debts due on band, and criminal of fences are not to be arbitrated.

ARBITRATOR, a private extraordinary judge, chosen by the mutual consent of parties, to determine controversies between them. Arbitrators are to award what is equal between both parties, and the performance must be lawful and possible. An action of debt may be brought for money adjudged to be paid by arbitrators.

ARBOR Dianæ. See CHEMISTRY.
ARBOR vitæ. See THUJA.

ARBOR, in mechanics, the principal part of a machine which serves to sustain the rest; also the axis or spindle on which a

machine turns, as the arbor of a crane, windmill, &c.

ARBUTUS, the strawberry-tree, in botany, a genus of the Decandria Monogynia class of plants, the calyx of which is a very small, obtuse, permanent perianthium, divided into five segments; the corolla consists of a single oval petal, divided also into five segments; the fruit is a roundish berry, containing five cells, and small osseous seeds. There are ten species.

ARC concentric, is that which has the same centre, with another arc.

ARC diurnal, that part of a circle described by a heavenly body, between its rising and setting; as the nocturnal arc is that described between its setting and rising: both these together are always equal.

ARCS equal, those which contain the same number of degrees, and whose radii are equal.

ARCA, in conchology, a genus of Bivalves, the animal of which is supposed to be a tethys, the valves are equal, and the hinge beset with numerous sharp teeth inserted between each other.

.

ARCA, in natural history, a genus of worms of the order Testacea; animal a tethys; shell bivalve, equivalve; hinge with numerous sharp teeth, alternately inserted between each other. There are, according to Gmelin, 43 species: but they are separated into four divisions, viz. A. margin very entire, beaks recurved; B. margin entire, beaks inflected; C. margin crenate, beaks recurved; D. margin crenate, beaks inflected: of the latter we shall notice A. nucleus; shell obliquely ovate, smoothish, with a triangular hinge; inhabits European seas, and is sometimes found fossile, the size of a hazel nut, covered with an olivaceous skin, under which it is white, within silvery; shell unequally triangular, with very fine perpendicular striæ, crossed by a few arched transverse ones; depression behind the beak, heart-shaped.

ARCH, or ARC, in geometry, any part of the circumference of a circle, or curved line, lying from one point to another, by which the quantity of the whole circle or line, or some other thing sought after, may be gathered.

All angles are measured by arcs. For this purpose an arc is described having its centre in the point or vertex of the angle: and as every circle is supposed to be divided into 360°, an arc is estimated according to the number of degrees which it contains.

Thus an arc is said to be of 30, 50, or 100 degrees, &c.

ARCH, in architecture, a concave building, with a mold bent in the form of a curve, erected to support some structure. Arches are either circular, elliptical, or straight, as they are improperly called by workmen. Circular arches are also of three kinds : 1. Semicircular, which have their centre in the middle of a line drawn betwixt the feet of the arch. 2. Scheme or skene, which are less than a semicircle, containing some 90, and some 70 degrees. 3. Arches of the third and fourth point, consisting of two arches of a circle meeting in an angle at the top, being drawn from the division of a chord into three or more parts at pleasure.

Elliptical arches consist of a semi-ellipsis, and have commonly a key-stone and imposts: they are usually described by workmen on three centres.

Straight arches are those used over doors and windows, having plain straight edges, both upper and under, which are parallel, but both the ends and joints point towards

a center.

The term arch is peculiarly used for the space between two piers of a bridge, intended for the passage of water, vessels, &c.

ARCH of equilibration, is that which is in equilibrium in all its parts, having no tendency to break in any one part more than in another; and which is, therefore, safer and stronger than any other figure. No other arch than this can admit of a horizontal line at top: it is of a form both graceful and convenient, as it may be made higher or lower at pleasure, with the same span.

All other arches require extrados that are curved, more or less, either upwards or downwards; of these, the elliptical arch approaches the nearest to that of equilibration for strength and convenience, and it is the best form for most bridges, as it can be made of any height to the same span, its haunches being at the same time sufficiently elevated above the water, even when it is very flat at top. Elliptical arches also appear bolder and lighter, are more uniformly strong, and are cheaper than most others, as they require less materials and labour. Of the other curves, the cycloidal arch is next in quality to the elliptical one, and lastly the circle.

ARCHANGEL, in botany. See LA.

MIUM.

ARCHES, or Court of ARCHES, the supreme court belonging to the Archbishop

of Canterbury, to which appeals fie from all the inferior courts within his province.

ARCHETYPE, the first model of a work, which is copied after to make another like it. Among minters it is used for the standard weight by which the others are adjusted. The archetypal world, among Platonists, means the world as it existed in the idea of God, before the visible creation.

ARCHIL. See LICHEN.

ARCHIMEDES, in biography, one of the most celebrated mathematicians among the ancients, who flourished about 250 years before Christ, being about 50 years later than Euclid. He was born at Syracuse in Sicily, and was related to Hiero, who was then king of that city. The mathematical genius of Archimedes set him with such distinguished excellence in the view of the world, as rendered him both the honour of his own age, and the admiration of posterity. He was indeed the prince of the ancient mathematicians, being to them what Newton is to the moderns, to whom in his genius and character he bears a very near resemblance. He was frequently lost in a kind of reverie, so as to appear hardly sensible; he would study for days and nights together, neglecting his food; and Plutarch tells us that he used to be carried to the baths by force. Many particulars of his life, and works, mathematical and mechanical, are recorded by several of the ancients, as Polybius, Livy, Plutarch, Pappus, &c. He was equally skilled in all the sciences, astronomy, geometry, mechanics, hydrostatics, optics, &c. in all of which he excelled, and made many and great inven

tions.

Archimedes, it is said, made a sphere of glass, of a most surprising contrivance and workmanship, exhibiting the motions of the heavenly boos in a very pleasing manner.

some

Many wonderful stories are told of his discoveries, and of his very powerful and curious machines, &c. Hiero once admiring them, Archimedes replied, these effects are nothing, "but give me," said he, 66 other place to fix a machine on, and I will move the earth." He fell upon a curious device for discovering the deceit which had been practised by a workman, employed by the said king Hiero to make a golden crown. Hiero, having a mind to make an offering to the gods of a golden crown, agreed for one of great value, and weighed out the gold to the artificer. After some time he brought the crown home of the full

weight; but it was afterwards discovered or suspected that a part of the gold had been stolen, and the like weight of silver substituted in its stead. Hiero, being angry at this imposition, desired Archimedes to take it into consideration, how such a fraud might be certainly discovered. While engaged in the solution of this difficulty, he happened to go into the bath; where observing that a quantity of water overflowed, equal to the bulk of his body, it presently occurred to him, that Hiero's question might be answered by a like method; upon which he leaped out, and ran homeward, crying out sunna! venne! I have found it out! I have found it out! He then made two masses, each of the same weight as the crown, one of gold and the other of silver; this being done, he filled a vessel to the brim with water, and put the silver mass into it, upon which a quantity of water overflowed equal to the bulk of the mass; then taking the mass of silver out he filled up the vessel again, measuring the water exactly, which he put in; this shewed him what measure of water answered to a certain quantity of silver. Then he tried the gold in like manner, and found that it caused a less quantity of water to overflow, the gold being less in bulk than the silver, though of the same weight. He then filled the vessel a third time, and putting in the crown itself, he found that it caused more water to overflow than the golden mass of the same weight, but less than the silver one; so that, finding its bulk between the two masses of gold and silver, and that in certain known proportions, he was able to compute the real quantities of gold and silver in the crown, and so manifestly discovered the fraud.

Archimedes also contrived many machines for useful and beneficial purposes; among these, engines for launching large ships; screw pumps, for exhausting the water out of ships, marshes or overflowed lands, as Egypt, &c. which they would do from any depth.

But he became most famous by his curious contrivances, by which the city of Syracuse was so long defended, when besieged by the Roman consul Marcellus ; showering upon the enemy sometimes long darts and stones of vast weight and in great quantities; at other times lifting their ships up into the air, that had come near the walls, and dashing them to pieces by letting them fall down again; nor could they find their safety in removing out of the reach of his cranes and levers, for there he

contrived to set fire to them with the rays of the sun reflected from burning glasses.

However, notwithstanding all his art, Syracuse was at length taken by storm, and Archimedes was so very intent upon some geometrical problem, that he neither heard the noise, nor regarded any thing else, till a soldier that found him tracing lines, asked his name, and upon his request to begone, and not disorder his figures, slew him.

"What gave Marcellus the greatest concern, says Plutarch, was the unhappy fate of Archimedes, who was at that time in his museum; and his mind, as well as his eyes, so fixed and intent upon some geometrical figures, that he neither heard the noise and hurry of the Romans, nor perceived the city to be taken. In this depth of study and contemplation, a soldier came suddenly upon him, and commanded him to follow him to Marcellus; which he refusing to do, till he had finished his problem, the soldier, in a rage, drew his sword, and ran him through." Livy says he was slain by a soldier, not knowing who he was, while he was drawing schemes in the dust; that Marcellus was grieved at his death, and took care of his funeral; and made his name a protection and honour to those who could claim a relationship to him. His death it seems happened about the 142d or 143d Olympiad, or 210 years before the birth of Christ.

When Cicero was quæstor for Sicily, he discovered the tomb of Archimedes, all overgrown with bushes and brambles; which he caused to be cleared, and the place set in order. There were a sphere and cylinder cut upon it, with an inscription, but the latter part of the verses were quite worn out.

Many of the works of this great man are still extant, though the greatest parts of them are lost. The pieces remaining are as follow: 1. Two books on the Sphere and Cylinder. 2. The Dimension of the Circle, or Proportion between the Diameter and the Circumference.—3. Of Spiral lines.-4. Of Conoids and Spheroids.—5. Of Equipon derants, or Centres of Gravity.-6. The Quadrature of the Parabola.—7. Of Bodies floating on Fluids.-8. Lemmata.-9. Of the Number of the Sand.

Among the works of Archimedes which are lost, may be reckoned the descriptions of the following inventions, which may be gathered from himself and other ancient anthors. 1. His Account of the Method which he employed to discover the Mixture of Gold and Silver in the Crown, mentioned by Vitruvius.-2. His Description of the Coch

leon, or engine to draw water out of places where it is stagnated, still in use under the name of Archimedes's Screw. Athenæus, speaking of the prodigious ship built by the order of Hiero, says, that Archimedes invented the cochleon, by means of which the hold, notwithstanding its depth, could be drained by one man. And Diodorus Siculus says, that he contrived this machine to drain Egypt, and that by a wonderful mechanism it would exhaust the water from any depth.-3. The Helix, by means of which, Athenæus informs us, he launched Hiero's great ship.-4. The Trispaston, which, according to Tzetzes and Oribasius, could draw the most stupendous weights.5. The Machines, which, according to Poły bius, Livy, and Plutarch, he used in the defence of Syracuse against Marcellus, consisting of Tormenta, Balistæ, Catapults, Sagittarii, Scorpions, Cranes, &c.-6. His Burning Glasses, with which he set fire to the Roman gallies.-7. His Pneumatic and Hydrostatic Engines, concerning which subjects he wrote some books, according to Tzetzes, Pappus, and Tertullian.-8. His Sphere, which exhibited the celestial motions. And probably many others.

A considerable volume might be written upon the curious methods and inventions of Archimedes, that appear in his mathematical writings now extant only. He was the first who squared a curvilineal space; unless Hypocrates be excepted on account of his lunes. In his time the conic sections were admitted into geometry, and he applied himself closely to the measuring of them, as well as other figures. Accordingly he determined the relations of spheres, spheroids, and conoids to cylinders and cones; and the relations of parabolas to rectilineal planes whose quadratures had. long before been determined by Euclid. He has left us also his attempts upon the circle : he proved that a circle is equal to a rightangled triangle, whose base is equal to the circumference, and its altitude equal to the radius; and consequently, that its area is equal to the rectangle of half the diameter and half the circumference; thus reducing the quadrature of the circle to the determination of the ratio between the diameter and circumference; which determination however has never yet been done. Being disappointed of the exact quadrature of the circle, for want of the rectification of its circumference, which all his methods would not effect, he proceeded to assign an useful approximation to it: this he effected by the