ATLANTIS ATOMS. 



323 



ATI.AHTI-S; among the ancients, the name of an 

 island in the Atlantic, of which vague accounts had 

 been received from ships which had ventured into 

 the ocean. Their descriptions of its situation were 

 very indefinite, and, as they placed it in a spot where 

 afterwards no island was found, it was supposed that 

 it had sunk. But some persons imagine that Phoeni- 

 cian or Carthaginian merchant-ships (as we know 

 happened to a Portuguese ship in the time of Colum- 

 bus), being driven Out of their course by storms and 

 currents, were forced over to the American coasts, 

 from which they afterwards fortunately returned to 

 their country ; and that, therefore, the island of A. 

 mentioned by Plato, as well as the great nameless 

 island spoken of by Diodorus, Pliny, and Arnobius, 

 was nothing more than what is now called America. 

 The most distinct account of the island of A. is in 

 Plato's Timceus. See Atlantica. 



ATLAS ; a chain of mountains whicli extends over 

 a large part of Northern Africa. The Greater A. 

 runs through the kingdom of Morocco, as far south 

 as Sahara, and is more than 11,000 feet high. The 

 Lesser A. extends from Morocco, towards the N. E., 

 to the northern coast The mythology of the Greeks 

 assigned this mountain to a Titan, son of Japetus 

 and Clymene. Jupiter, the conqueror of the Titans, 

 condemned him to bear the vault of heaven ; which 

 fable arose from his lofty stature. He was endowed 

 with wisdom, and later accounts ascribe to him much 

 knowledge, particularly of astronomy. By Pleione, 

 the daughter of Oceanus, lie had seven daughters, 

 who, under the name of Pleiades (called, likewise, 

 after their father, Atlantides), shone in the heavens. 

 According to some, he was also the father of the 

 Hyades. Atlas, in anatomy, is the name of the first 

 vertebra of the neck, which supports the head At- 

 las, in commerce ; a silk cloth manufactured in the 

 East Indies. The manufacture is admirable, and, 

 as yet, inimitable by Europeans ; yet it has not that 

 lustre, which the French know how to give to their 

 silk stuffs. Atlas, a name given to collections of 

 maps and charts ; so called from the giant who sup- 

 ported heaven. This name was first used to signify 

 a geographical system, by Gerard Mercator, in the 

 16th century. 



ATMOSI-HERE; commonly, the air in which our 

 earth appears to swim ; but, in the widest sense, it is 

 lliat mass of thin, elastic fluid, with which any body 

 is completely surrounded. Hence we speak of an 

 atmosphere of the sun, of the moon, of the planets, 

 of electric and magnetic bodies, c., the existence of 

 which may not be fully proved, but is more or less 

 probable. It is certain that our earth has an atmos- 

 phere, by which, according to the preceding defini- 

 tion, we understand the surrounding body of air and 

 vapour. By means of its weight, the air is insepara- 

 bly connected with the earth, and presses on it ac- 

 cording to the laws of heavy, elastic fluids. Its 

 whole pressure is equal to its weight, and, like that 

 of all other heavy, elastic fluids, is exerted equally 

 on all sides. If, now, by any circumstance, a stronger 

 pressure is exerted on one side, certain phenomena 

 are observed, whicli continue till the equilibrium is 

 restored. Thus, for instance, water ascends, in the 

 bore of a pump, above its general level, as soon as a 

 vacuum is made between it and the piston, which is 

 drawn tip. The cause of this is the disturbance of 

 the equilibrium, since the air without the bore presses 

 on the water without, while no air is present within. 

 By means of this pressure, if the bore is long enough, 

 the water may be raised to the height of thirty-two 

 and a half feet. This is the weight with which the 

 atmosphere presses on the earth, and which is equal 

 to the pressure of an ocean thirty-two and a half feet 

 deep, spread o\tr the whole earth. Hence it fol- 



lows, that, at twenty-eight inches barometrical 

 height, the atmosphere presses with a weight of 

 32,440 pounds on the human body, estimated at 

 fifteen square feet. The man does not perceive this 

 pressure, because the air entirely surrounds him, and 

 is, besides, within him. On account of its elasticity, 

 it presses in every direction, even from within the 

 man outwards, and consequently counterbalances the 

 air spread over the body. That the atmosphere has 

 not a uniform density, may be inferred from this, that 

 the lower strata of the air have to support the weight, 

 of the upper ones, on which account they must be- 

 come more compressed and denser. According to 

 the law of Mariotte, the density of the atmosphere 

 diminishes in geometrical, while the height increases 

 in arithmetical progression. This law may not hold 

 at the extreme limits of the atmosphere, because the 

 air at that height, free from all pressure, must be 

 completely in its natural state. The height of the 

 atmosphere has been estimated, by natural philoso- 

 phers, at from thirty to forty miles partly from the 

 pressure which it exerts, partly from the twilight ; 

 since it is to be supposed, that the air, as far as it 

 reflects light or receives illumination, belongs to our 

 planet. Delambre, however (Astronomic, vol. iii., p. 

 337), considers this height to be almost forty-six 

 miles, which, remarkably enough, Kepler has men- 

 tioned in the Cap. Astr. , p. 73. According to the ad- 

 mirable paper of Dr Wollaston, on the limits of the 

 atmosphere, the height cannot be less than forty, 

 or more than fifty miles. (See Phil. Trans.) In 

 respect to its form, the atmosphere may be con- 

 sidered as a spheroid, elevated at the equator, on ac- 

 count of the diurnal motion of the earth, and also on 

 account of the great rarefaction of the air by the 

 sun's rays, which there exert a powerful influ- 

 ence. The constituent parts of the earth's atmos- 

 phere are nitrogen and oxygen, which are found 

 every where, ana at all times, nearly in the propor- 

 tion of 79 to 21. Beside these, there is a small por- 

 tion of carbonic acid, a variable portion of aqueous 

 vapour, and a very small, indefinite quantity of hydro- 

 gen. (See Gas.) It also contains, in the form of 

 vapour, a multitude of adventitious substances, in 

 those injurious mixtures known under the name miaa 

 wate,the nature of which can hardly be investigated. 

 As to the manner in which these different ingredients 

 are united, various hypotheses have been formed, of 

 which that of Dalton, which denies a chemical mix- 

 ture, is one, of the most celebrated, but also the most 

 opposed. See De Luc's Recherches sur Ics Modifi- 

 cations de I' Atmosphere, 2 vols. 4to., Geneva, 1772 (in 

 German, Leipsic, 1776 78), and the section a" Atmos- 

 phere, in Biot's Traite d 'Astronomic Physique, 2d ed., 

 Paris, 1810, 3 vols. On the atmosphere of the sun, 

 moon, and the other planets, see the respective 

 articles. See, also, Air. 



ATOMS; according to the hypothesis of some 

 philosophers, the primary parts of elementary matter 

 not any further divisible. Moschus of Sidon, who is 

 said to have lived before the Trojan war, taught, as 

 we are told, that the original matter is composed of 

 small, indivisible bodies. Leucippus (510 B. C.) 

 established a system respecting the origin of the 

 world, resting on the mixture of atoms, in which 

 chance governed, in opposition to the immaterial 

 system of the Eleatics, who contended, that whatever 

 existed was only one being, and that all apparent 

 changes in the universe are mere, illusions of sense. 

 Democritus and Epicurus extended this system : the 

 latter, particularly, made many additions to it. Lu- 

 cretius, and, among the moderns, Gassendi, have- 

 illustrated the doctrine of Epicurus. Descartes form- 

 ed from this his system of the vortices. Newton and 

 Boerliaave supposed that the original matter consists 



