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 of Constantinople to the emperor. The rivalry of the see of Alexandria with Constantinople was also displayed in the contest, Theophilus, patriarch of Alexandria, assisting the court in bringing about the fall of Chrysostom. Throughout the reign of Arcadius there was estrangement and jealousy between the two brothers or their governments. The principal ground of this hostility was probably dissatisfaction on both sides with the territorial partition. The line had been drawn east of Dalmatia. The ministers of Arcadius desired to annex Dalmatia to his portion, while the general Stilicho, who was supreme in the west, wished to wrest from the eastern realm the prefecture of Illyricum or a considerable part of it. His designs were unsuccessful, and during the reign of Theodosius II., son of Arcadius (who died in 408), Dalmatia was transferred to the dominion of the eastern ruler.

ARCADIUS, of Antioch, Greek grammarian, flourished in the 2nd century According to Suidas, he wrote treatises on orthography and syntax, and an onomaticon (vocabulary), described as a wonderful production. An epitome of the great work of Herodian on general prosody in twenty books, wrongly attributed to Arcadius, is probably the work of Theodosius of Alexandria or a grammarian named Aristodemus. This epitome ( ) only includes nineteen books of the original work; the twentieth is the work of a forger of the 16th century. Although meagre and carelessly put together, it is valuable, since it preserves the order of the original and thus affords a trustworthy foundation for its reconstruction.

ARCELLA (C. G. Ehrenberg), a genus of lobose Rhizopoda, characterized by a chitinous plano-convex shell, the circular aperture central on the flat ventral face, and more than one nucleus and contractile vacuole. It can develop vacuoles, or rather fine bubbles of carbonic acid gas in its cytoplasm, to float up to the surface of the water. ARCESILAUS (316–241 ), a Greek philosopher and founder of the New, or Middle, Academy (see ). Born at Pitane in Aeolis, he was trained by Autolycus, the mathematician, and later at Athens by Theophrastus and Crantor, by whom he was led to join the Academy. He subsequently became intimate with Polemon and Crates, whom he succeeded as head of the school. Diogenes Laërtius says that he died of excessive drinking, but the testimony of others (e.g. Cleanthes) and his own precepts discredit the story, and he is known to have been much respected by the Athenians. His doctrines, which must be gathered from the writings of others (Cicero, Acad. i. 12, iv. 24; De Orat. iii. 18; Diogenes Laërtius iv. 28; Sextus Empiricus, Adv. Math. vii. 150, Pyrrh. Hyp. i. 233), represent an attack on the Stoic  (Criterion) and are based on the sceptical element (see ) which was latent in the later writings of Plato. He held that strength of intellectual conviction cannot be regarded as valid, inasmuch as it is characteristic equally of contradictory convictions. The uncertainty of sensible data applies equally to the conclusions of reason, and therefore man must be content with probability which is sufficient as a practical guide. “We know nothing, not even our ignorance”; therefore the wise man will be content with an agnostic attitude. He made use of the Socratic method of instruction and left no writings. His arguments were marked by incisive humour and fertility of ideas.

ARCH, JOSEPH (1826–&emsp;&emsp;), English politician, founder of the National Agricultural Labourers’ Union, was born at Barford, a village in Warwickshire, on the 10th of November 1826. His parents belonged to the labouring class. He inherited a strong sentiment of independence from his mother; and his objections to the social homage expected by those whom the catechism boldly styled his “betters” made him an “agitator.” Having educated himself by unremitting exertions, and acquired fluency of speech as a Methodist local preacher, he founded in 1872 the National Agricultural Labourers’ Union, of which he was president. A rise then came in the wages of agricultural labourers, but this had the unforeseen effect of destroying the union; for the labourers, deeming their object gained, ceased to “agitate.” Mr Arch nevertheless retained sufficient popularity to be returned to parliament for north-west Norfolk in 1885; and although defeated next year owing to his advocacy of Irish Home Rule, he regained his seat in 1892, and held it in 1895, retiring in 1900. He was deservedly respected in the House of Commons; seldom has an agitator been so little of a demagogue.

ARCH, in building, a constructional arrangement of blocks of any hard material, so disposed on the lines of some curve that they give mutual support one to the other.

The blocks, which are technically known as voussoirs, should be of a wedge shape, the centre or top block (see fig. 1, A) being the keystone A; the lower blocks B B which rest on the supporting pier are the springers, the upper surface of which is called the skewback, C C; the side blocks, as D, are termed the haunches. The lower surface or soffit of the arch is the intrados, E, and the upper surface the extrados, F. The rise of the arch is the distance from the springing to the soffit, G, the width between the springers is called the span, H, and the radius I. The triangular spaces between the arches are termed spandrils, K.

The arch is employed for two purposes:—(1) to span an opening in a wall and support the superstructure; (2) when continuous to form a vault known as a barrel or waggon vault.

The arch has been used from time immemorial by every nation, but owing to the tendency of the upper portion to sink, especially when bearing any superincumbent weight, it requires strong lateral support, and it is for this reason that in the earliest examples in unburnt brick at Nippur in Chaldaea, c. 4000, and at Rakākna (Requaqna) and Dendera in Egypt, 3500–3000 , it was employed only below the level of the ground which served as an abutment on either side.

In the building of an arch, the voussoirs have to be temporarily