ARfc 



ARC 



he United States, and is in many places 

 called Jiloody Clam; when opened the in- 

 cluded liquid has a dirty red appearance ; 

 shell obliquely heart-shaped, with numer- 

 ous unarmed grooves; it is white, but cov- 

 ered with a brownish hairy skin : the an- 

 terior slope with a compressed prominent 

 angle, 



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 di- 

 vided into 360, an arc is estimated ac- 

 cording 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 build- 

 ing, with a mould bent in the form of a 

 curve, erected to support some structure. 

 Arches are either circular, elliptical, or 

 straight, us they are improperly called by 

 workmen. Circular arches are also of 

 thre: kinds : 1. Semicircular, which have 

 their centre in the middle of aline 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 meet- 

 Ing 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-ellip- 

 sis, 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 centre. 



The term arch is peculiarly used for the 

 space between two piers of a bridge, in- 

 tended 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 be- 

 ing at the same time sufficiently elevated 

 above the water, even when it is very flat 

 at top. Elliptical arches also appear bold- 

 er and lighter, are more uniformly strong, 

 and tire 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- 

 Mirsr. 



ARCHES, or Court of ARCHES, the 

 supreme court belonging to the Arch- 

 bishcn of Canterbury, to which appeals 

 lie from all the inferior courts within his 

 province. 



ARCHETYPE, the first model of a 

 work which is copied after, to make ano- 

 ther 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 vi- 

 sible creation. 



ARCHIL. See LICIIE*. 



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 excel- 

 lence in the view of the world, as render- 

 ed him both the honour of his own age, 

 and the admiration of posterity. He was 

 indeed the prince of the ancient mathe- 

 maticians, being to them what Newton is 

 to the moderns, to whom in his genius 

 and character he bears a very near re- 

 semblance. 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 car- 

 ried to the baths by force. Many parti- 

 culars of his life, and works, mathemati- 

 cal and mechanical, are recorded by se- 

 veral of the ancients, as Polybius, Livy, 

 Plutarch, Pappus, &c. He was equally 

 skilled in all the sciences, astronomy, 

 geometry, mechanics, hydrostatics, op- 

 tics, &c. in all of which he excelled, and 

 made many and great inventions. 



Archimedes, it is said, made a sphere 

 of glass, of a most surprising contrivance 



