48 A B E A B I B travel from the star to the earth. The effect of this com bination of motions may be best explained by a familiar illus tration. Suppose a rain-drop falling vertically is received in a tube that has a lateral motion. In order that the drop may fall freely down the axis of the tube, the hitter must be inclined at such an angle as to move from the position AD to BE, and again to OF, in the times the drop moves from D to G, and from G to C. The drop in this case, since it moves down the axis all the way, must strike the bottom of the tube at C in the direction FC. The light proceeding from a star is not seen in its true direc tion, but strikes the eye obliquely, for a precisely similar reason. If lines be taken to represent the motions, so that the eye is carried from A to C during the time that light moves from D to C, the Light will appear to the eye at C to come, not from D, but from F. The angle DCF, con tained by the true and apparent directions of the star, is the aberration. It is greatest when the two motions are at right angles to each other, i.e., when the star s longitude is 90 in advance of, or behind, the heliocentric longitude of the earth, or (which amounts to the same thing) 90 behind, or in advance of, the geocentric longitude of the sun. (See ASTRONOMY.) Now, in the right-angled triangle AC ACD, tan ADC (i.e., DCF) = ^-^ ; whence it appears that the tangent of the angle of aberration (or, since the angle is very small, the aberration itself) is equal to the ratio, velocity of earth in orbit velocity of light Thc rate of the eartlis motlon being to the velocity of light in the proportion of 1 to 10,000 nearly, the maximum aberration is small, amount ing to about 20 4 seconds of arc, a quantity, however, which is very appreciable in astronomical observations. Aberration always takes place in the direction of the earth s motion ; that is, it causes the stars to appear nearer than they really are to the point towards which the earth is at the moment moving. That point is necessarily on the ecliptic, and 90 in advance of the earth in longitude. The effect is to make a star at the pole of the ecliptic appear to move in a plane parallel to the ecliptic, so as to form a small ellipse, similar to the earth s orbit, but having its major axis parallel to the minor axis of that orbit, and vice versa. As we proceed from the pole, the apparent orbits the stars describe become more and more elliptical, till in the plane of the ecliptic the apparent motion is in a straight line. The length of this line, as well as of the major axes of the different ellipses, amounts, in angular measure, to about 40" 8. The stars thus appear to oscil late, in the course of the year, 20" 4 on each side of their true position, in a direction parallel to the plane of the ecliptic, and the quantity 20" 4 is therefore called the constant of aberration. For the discovery of the aberration of light, one of the finest in modern astronomy, we are indebted to the dis tinguished astronomer Dr Bradley. He was led to it, in 1727, by the result of observations he made with the view of determining the annual parallax of some of the stars ;. that is, the angle subtended at these stars by the diameter of the earth s orbit. He observed certain changes in the positions of the stars that he could not account for. The deviations were not in the direction of the apparent motion that parallax would give rise to; and he had no better success in attempting to explain the phenomenon by the nutation of the earth s axis, radiation, errors of observa tion, &c. At last the true solution of the difficulty occurred t~- him, suggested, it is said, by the movements of a vane on the top of a boat s mast. Roemer had discovered, a quarter of a century before, that light has a velocity which admits of measurement; and Bradley perceived that the earth s motion, having a perceptible relation to that of light, must aifect the direction of the visual rays, and with this the apparent positions of the stars. He calculated the aberration from the known relative velocities of the earth and of light, and the results agreed entirely with his observations. The observed effects of aberration are of importance as supplying an independent method of measuring the velocity of light, but more particularly as presenting one of the few direct proofs that can be given of the earth s motion round the sun. It is indeed the most satisfactory proof of this that astronomy furnishes, the phenomenon being quite in explicable on any other hypothesis. ABERYSTWITH, a municipal and parliamentary bo rough, market town, and seaport of Wales, in the county of Cardigan, is situated at the western end of the Vale of Rheidol, near the confluence of the rivers Ystwith and Rheidol, and about the centre of Cardigan Bay. It is the terminal station of the Cambrian Railway, and a line to the south affords direct communication with South Wales, Bristol, &c. The borough unites with Cardigan, Lampeter, &c., in electing a member of Parliament. Coal, timber, and lime are imported, and the exports are lead, oak bark, flannel, and corn. The harbour has of late been much improved; and the pier, completed in 1805, forms an excellent promenade. There are many elegant build ings, and it has been proposed to establish here a Uni versity College of Wales. On a promontory to the S.W. of the town are the ruins of its ancient castle, erected in 1277, by Edward I., on the site of a fortress of great strength, built by Gilbert de Strongbow, and destroyed by Owen Gwynedd. From its picturesque situation and healthy climate, and the suitableness of the beach for bathing, Aberystwith has risen into great repute as a watering-place, and attracts many visitors. Much of the finest scenery in Wales, such as the Devil s Bridge, &c., lies within easy reach. Population (1871), G898. ABETTOR, a law term implying one who instigates, encourages, or assists another to perform some criminal action. See ACCESSORY. ABEYANCE, a law term denoting the expectancy of an estate. Thus, if fends be leased to one person for life, with reversion to another for years, the remainder for years is in abeyance till the death of the lessee.
ABGAR, the name or title of a line of kings of Edessa in Mesopotamia. One of them is known from a correspondence he is said to have had with Jesus Christ. The letter of Abgar, entreating Jesus to visit him and heal him of a disease, and offering Him an asylum from the wrath of the Jews, and the answer of Jesus promising to send a disciple to heal Abgar after His ascension, are given by Eusebius, who believed the documents to be genuine. The same belief has been held by a few moderns, but there can be no doubt whatever that the letter of Jesus at least is apocryphal. It has also been alleged that Abgar possessed a picture of Jesus, which the credulous may see either at Rome or at Genoa. Some make him the possessor of the handkerchief a woman gave Jesus, as He bore the cross, to wipe the sweat from His face with, on which, it is fabled, His features remained miraculously imprinted.
ABIAD, BAHR-EL-, a name given to the western branch of the Nile, above Khartoum. It is better known as tlio White Xile. See NILE.
