Armenian Era.—This starts from the Coun cil of Tiben, 9 July 552, when the Armenian schism began by condemnation of the acts of the Council of Chalcedon. The Armenian civil year is the Egyptian year of 365 days, out of all measure with others; their ecclesiastical year begins 11 August, and has the Julian cal endar. They also use the Creation era of Constantinople along with their own, dating documents with both on occasion. In busi ness with Europeans they employ our era and our year. To convert their civil dates to our era, multiply the number of Armenian years by 365• add the number of days from 1 January to the given date; subtract 176; the remainder will be the number of days from 1 Jan. 553 to the given date; reduce this to Julian years, add 552, and this gives the date in the Julian year, old style. Add the requisite number in the Gregorian calendar if desired. In ecclesiastical reckonings, add 551 years 223 days. In leap years, if the date is between 1 March and 10 August, subtract one day from the above.
Mohammedan Era.— This commemorates the Hijra or Hegira (flight) of the Prophet from Mecca to Medina, 622 A.D. It does not, however, date from the flight, but from 68 days previous, 16 July, or as some have it, 15 Jul'. The methods of computation are too compli cated and uncertain to be given here: Moham medan chronology is one of the most treacher ous of subjects, and even experts disagree and go astray.
Persian or Gelalaan Era.—This begins with the accession of Yezdegird III to the throne of Persia, 16 June 632. Till 1079 the Persian had the random Egyptian year, 365 days without intercalation; it was then re formed by Omar Khayyam, the great poet and astronomer, under Malek Shah, to a degree al most as accurate as the Gregorian. There were seven successive leap years once in four years, but the eighth was deferred till the fifth year. This method was once universal in Persia, and is still followed by the Parsecs of India. Owing to the days of error accumulated by the old year, the day of beginning is thrown back, so that the Persian year can be con verted into ours by adding 631.
Hindu Eras.— The philosophers divided the duration of the world into four yugs or ages, of 'which three are past, and the present cor rupt one, the is alone historical It begins 3101 B.C. and includes several others in use. The Era of Vikramaditya, from a Hindu Augustus who may or may not be historical, is rened from 57 B.C. This monarch is gen erally attributed to the 5th or 6th century if real, and the epoch is thought a sidereal one. The Era of Salivahana is 78 A.D. This is used in southern India, and commemorates an equally dubious ruler. The Fusli era, used in revenue transactions all over India, is not uni form, but the most usual begins 590 A.D. ; it
seems to be a Mohammedan one, and to corre spond roughly to the Hegira. The Bengali era is another of the sort, and is set at 631. There are also others which are .used in different provinces. The '60-year cycle is employed, its date being variously set.
Chinese Chronology.— This rests on cycles of 60 years, the mathematical tribunal fixing their start at 2277 B.C. But since 163 ac. the Chinese writers have used periods called hao, each beginning with the accession of some emperor, and named after him, as with English laws, and ending when he or some successor to begin a new period. Tables of hao are therefore needed 'to identify the years. FORREST MORGAN, Connecticut Historical Society. CHRONOMETER (Gr. xpdvoc, librpor, gmeasure=a- time-measuree), an ment designed to measure time with great curacy, and much used in scientific work, for the determination of longitude at sea, and in the regulation of clocks and watches by makers and repairers. The marine chronometer bears a strong superficial resemblance to a watch, cept that it is larger and is mounted upon bals, so that the motion of the ship may affect it as little as possible. In its mechanism it fers from a watch chiefly in the design of its escapement and balance-wheel. The usual form of escapement, as invented' by Le Roy about 1765 and greatly improved by Earnshaw and Arnold some 15 years later, is shown in Fig. 1. It is known as the °detached* or ode escapement. In the figure a spring de tent is shown, the spring being at F. This type is the usual one for marine chronometers. For pocket 'chronometers the spring is omitted -and the detent blade G is pivoted.
' In the ordinary watch the balance-wheel is In connection with the train of wheels leading from the mainspring continuously, or nearly 'so; hut in the chronometer it is free from these 'during the greater part of the time, so that its natural period of vibration is more nearly real ized. A is the escape-wheel, which is in gear with the other wheels of the instrument and is prevented from free rotation by _means of the locking-pallet, D. B is a disc (technically known as the "impulse-roller") secured to the same staff, or axis, as the balance-wheel. F is the "detent spring," which serves to return the light frame, G and the locking-pallet, D to the position they have in the illustration, after the detent has been tripped by the balance-wheel. A set screw, is provided for preventing the locking-pallet, D from being drawn too far into the escape-wheel, but this has been omitted in the illustration for the sake of greater clear ness.