ADVENT

, Adventus, in the Calendar, the time immediately preceding Christmas; and was anciently employed in pious preparation for the adventus, or coming on, of the feast of the Nativity.

Advent includes four Sundays, or weeks; commencing either with the Sunday which falls on St. Andrew's day, namely the 30th day of November, or the nearest Sunday to that day, either before or after.

ÆOLIPILE, Æolipile, in Hydraulics, a hollow ball of metal, with a very small hole or opening; chiefly used to shew the convertibility of water into elastic steam. The best way of fitting up this instrument, is with a very slender neck or pipe, to screw on and off, for the convenience of introducing the water into the inside; for by unscrewing the pipe, and immerging the ball in water, it readily fills, the hole being pretty large; and then the pipe is screwed on. But if the pipe do not screw off, its orifice is too small to force its way in against the included air; and therefore to expel most of the air, the ball is heated red hot, and suddenly plunged with its orifice into water, which will then rush in till the ball is about two-thirds filled with the water. The water having been introduced, the ball is set upon the fire, which gradually heats the contained water, and converts it into elastic steam, which rushes out by the pipe with great violence and noise; and thus continues till all the water is so discharged; though not with a constant and uniform blast, but by sits: and the stronger the fire is, the more elastic will the steam be, and the force of the blast. Care should be taken that the ball be not set upon a violent fire with very little water in it, and that the small pipe be not stopped with any thing; for in such case, the included elastic steam will suddenly burst the ball with a very dangerous explosion.

This instrument was known to the ancients, being mentioned by Vitruvius, lib. 1. cap. 6. It is also treated of, or mentioned, by several modern authors, as Descartes, in his Meteor. cap. 4; and Father Mersennus, in prop. 29 Phædom. Pneumat. uses it to weigh the air, by first weighing the instrument when red hot, and having no water in it; and afterwards weighing the same when it becomes cold. But the conclusion gained by this means, cannot be quite accurate, as there is supposed to be no air in the ball when it is red hot; whereas it is shewn by Varenius, in his Geography, cap. 19, sect. 6, prop. 10, that the air is raresied but about 70 times; and consequently the weight obtained by the above process, will be about one-70th too small, or more or less according to the intensity of the heat.

In Italy it is said that the Æolipile is often used to cure smoaky chimneys: for being hung over the fire, the blast arising from it carries up the loitering smoke along with it.

And some have imagined that the æolipile might be employed as bellows to blow up a fire, having the blast from the pipe directed into the fire: but experience would soon convince them of their mistake; for it would rather blow the sire out than up, as it is not air, but rarefied water, that is thus violently blown through the pipe.

ÆOLUS, in Mechanics, a small portable machine, not long since invented by Mr. Tidd, for refreshing and changing the air in rooms which are made too close.

The machine is adapted to supply the place of a | square of glass in a sash-window, where it works with little or no noise, on the principle of the sails of a mill, or a smoke-jack; and thus admitting an agreeable quantity of air, at a convenient part of the room.

ÆOLUS's Harp, or Æolian Harp, an instrument so named, from its producing an agreeable melody, merely by the action of the wind.

Neither the age nor inventor of this instrument are very well known. It is not mentioned by Mersennus in his Harmonics, where he describes most sorts of musical instruments: and yet the description and use of it was given soon after, by Kircher, in his book, Magia Phenotactica & Phonurgia.

The construction of this instrument is thus; let a box be made of as thin deal as possible, its length answering exactly to the width of the window in which it is to be placed; five or six inches deep, and seven or eight inches wide. Across the top, and near each end, glue on a bit of wainscot, about half an inch high, and a quarter of an inch thick, to serve as two bridges for the strings to be stretched over, by means of pins inserted into holes a little behind the bridges, nearer the ends, half the number being at one end, and half at the other end: these pins are like those of a harpsichord; and for their better support in the thin deal, a piece of beech of about an inch square, and length equal to the breadth of the box, is glewed on the inside of the lid, immediately under the place of the pins, the holes for receiving them being bored through this piece. It is strung with small catgut, or blue first fiddle strings, more or less at pleasure, on the outside and lengthways of the lid, fixing one end to one of the small pins, and twisting the other end about the opposite or stretching pin. A couple of sound-holes are cut in the lid; and the thinner this is, the better will be the performance.

When the strings are tuned unison, and the instrument placed, with the top or stringed side outwards, in the window to which it is fitted, the air blowing upon that window, the instrument will give a sound like a distant choir, increasing or decreasing according to the strength of the wind.

ÆRA, in Chronology, is the same as epoch, or epocha, and means a sixed point of time, from which to begin a computation of the years ensuing.

The word is sometimes also written cra in ancient authors. Its origin is contested, though it is generally supposed that it had its rise in Spain. Some imagine that it is formed from a. er. a. the abbreviations of the words, annus erat Augusti, or from a. e. r. a. the initials of the words annus erat regni Augusti, because the Spaniards began their computation from the time that their country came under the dominion of Augustus. Others derive it from æs, brass, the tribute money with which Augustus taxed the world. It is also said that æra originally signified a number stamped on money to determine its current value. And that the ancients used æs or æra as an article, as we do the word item, to each particular of an account; and hence it came to stand for a sum or number itself.

ÆRA also means the way or mode of accounting time. Thus we say such a year of the Christian æra, &c.

Spanish ÆRA, otherwise called the year of Cæsar, was introduced after the second division of the Roman provinces, between Augustus, Anthony, and Lepidus, in the 714th year of Rome, the 4676th year of the Julian period, and the 38th year before Christ. In the 447th year of this æra, the Alani, the Vandals, Suevi, &c, entered Spain. It is frequently mentioned in the Spanish affairs; their councils, and other public acts, being all dated according to it. Some say it was abolished under Peter IV, king of Arragon, in the year of Christ 1358, and the Christian æra introduced instead of it. But Mariana observes that it ceased in the year of Christ 1383, under John I, king of Castile. The like was afterwards done in Portugal.

Christian ÆRA. It is generally allowed by Chronologers, that the computation of time from the birth of Christ, was only introduced in the sixth century in the reign of Justinian; and it is commonly ascribed to Dionysius Exiguus. This æra came then into use in deeds, and such like; before which time either the olympiads, the year of Rome, or that of the reign of the emperors, was used for such purposes.

See an account of the other principal æras under the word Epoch.

AERIAL Perspective, is that which represents bodies diminished and weakened, in proportion to their distance from the eye.

Aërial Perspective chiefly respects the colours of objects, whose force and lustre it diminishes more or less, to make them appear as if more or less remote.

It is founded upon this, that the longer the column of air an object is seen through, the more feebly do the visual rays emitted from it affect the eye.