ATALANTIS, or ATLANTICA, an island mentioned by Plato and some others of the ancients, concerning the real existence of which many disputes have been raised. Homer, Horace, and the other poets make two Atlanticas, calling them Hesperides and Elysian Fields, making them the habitations of the blessed. The most distinct account of this island we have in Plato's Timaeus, of which Mr Chambers gives the following abridgment. "The Atlantis was a large island in the western ocean, situated before or opposite to the straits of Gades. Out of this island there was an easy passage into some others, which lay near a large continent exceeding in bigness all Europe and Asia. Neptune settled in this island (from whose son Atlas its name was derived), and divided it among his ten sons. To the youngest fell the extremity of the island, called Gadir, which in the language of the country signifies fertile, or abundant in sheep. The descendants of Neptune reigned here from father to son, for a great number of generations, in the order of primogeniture, during the space of 9000 years. They also possessed several other islands; and passing into Europe and Africa, subdued all Libya as far as Egypt, and all Europe to Asia Minor. At length the island sunk under water; and for a long time afterwards the sea thereabouts was full of rocks and shelves."

Many of the moderns also are of opinion, that the existence of the Atlantis is not to be looked upon as entirely fabulous. Some take it to have been America; and from thence, as well as from a passage in Seneca's Medea, and some other obscure hints, they imagine that the new world was not unknown to the ancients. But allowing this to be the case, the above-mentioned continent, which was said to lie beyond Atlantis, Atlantis, would seem rather to have been the continent of America than Atlantis itself. The learned Rudbeck, professor in the university of Upsal, in a work entitled *Atlantica sive Manheim*, endeavours to prove that Sweden and Norway are the Atlantis of the ancients; but this its situation will by no means allow us to believe. By Kircher it is supposed to have been an island extending from the Canaries quite to the Azores; that it was really swallowed up by the ocean, as Plato asserts; and that these small islands are the shattered remains of it which were left standing.

**Atlantis, New**, is the name of a fictitious philosophical commonwealth, of which a description has been given by Lord Bacon. The New Atlantis is supposed to be an island in the South sea, to which the author was driven in a voyage from Peru to Japan. The composition is an ingenious fable, formed after the manner of the Utopia of Sir Thomas More, or Campanella's City of the Sun. Its chief design is to exhibit a model or description of a college, instituted for the interpretation of nature and the production of great and marvellous works, for the benefit of men, under the name of Solomon's House, or "the college of the six days work." This much, at least, is finished; and with great beauty and magnificence. The author proposed also a frame of laws, or of the best state or mould of a commonwealth. But this part is not executed.

**Atlas**, king of Mauritania, a great astronomer, contemporary with Moses. From his taking observations of the stars from a mountain, the poets feigned him to have been turned into a mountain, and to sustain the heavens on his shoulders. Being an excellent astronomer, and the first who taught the doctrine of the sphere, they tell us that his daughters were turned into stars: seven of them forming the Pleiades, and other seven the Hyades.

**Atlas**, a chain of mountains in Africa, lying between the 20th and 25th degree of north latitude, and supposed almost to divide the continent from east to west. They are said to have derived their name from Atlas king of Mauritania, who was a great astronomer. They are greatly celebrated by the ancients on account of their height, insomuch that the above-mentioned king, who is said to have been transformed into a mountain, was feigned to bear up the heavens on his shoulders. We are assured, however, by Dr Shaw, that the part of this chain of mountains which fell under his observation could not stand in competition either with the Alps or Apennines. He tells us, that if we conceive a number of hills, usually of the perpendicular height of 400, 500, or 600 yards, with an easy ascent, and several groves of fruit or forest trees, rising up in a succession of ranges above one another; and that if to this prospect we add now and then a rocky precipice, and on the summit of each imagine a miserable mud-walled village; we shall then have a just idea of the mountains of Atlas.

According to M. Chenier, this mountain is formed by an endless chain of lofty eminences, divided into different countries, inhabited by a multitude of tribes, whose ferocity permits no stranger to approach. "I have not been able (continues he) to obtain a sufficient knowledge of these mountains to describe them accurately: What Leo Africanus has said of them is very vague; and his account is the less to be regarded at present, as it is now about three centuries since he wrote, and the face of the country has been in that time totally changed. Nothing perhaps would be more interesting to the curiosity of the philosopher, or conducing more to the improvement of our knowledge in natural history, than a journey over Mount Atlas. The climate, though extremely cold in winter, is very healthy and pleasant; the valleys are well cultivated, abound in fruits, and are diversified by forests and plentiful springs, the streams of which uniting at a little distance, form great rivers, and lose themselves in the ocean. According to the reports of the Moors, there are many quarries of marble, granite, and other valuable stone, in these mountains: It is probable there are also mines, but the inhabitants have no idea of these riches; they consider their liberty, which their situation enables them to defend, as the most inestimable of all treasures."

**Atlas**, in matters of literature, denotes a book of universal geography, containing maps of all the known parts of the world.

**Atlas**, in Commerce, a silk-satin, manufactured in the East Indies. There are some plain, some striped, and some flowered, the flowers of which are either gold or only silk. There are atlases of all colours, but most of them false, especially the red and the crimson. The manufacture of them is admirable; the gold and silk being worked together after such a manner as no workman in Europe can imitate; yet they are very far from having that fine gloss and lustre which the French know how to give to their silk stuffs. In the Chinese manufactures of this sort, they gild paper on one side with leaf-gold; then cut it in long slips, and weave it into their silks; which makes them with very little cost, look very rich and fine. The same long slips are twisted or turned about silk threads, so artificially, as to look finer than gold thread, though it be of no greater value.

**Atmosphere**, a word generally used to signify the whole mass of fluid, consisting of air, aqueous and other vapours, electric fluid, &c., surrounding the earth to a considerable height.

The composition of that part of our atmosphere improperly called air, was till lately very much unknown. Former times it was supposed to be a simple, homogeneous and elementary fluid. The experiments of Dr Priestley discovered, that the purest kind of air, which he called dephlogisticated, was in reality a compound, and might be artificially produced in various ways. His first conjectures concerning its component parts were, that it consisted of earth, nitrous acid, and phlogiston. Subsequent experiments rendered these conjectures dubious; and at last it was supposed that dephlogisticated air is a pure elementary substance, the vivifying principle to animals, and the acidifying principle throughout all nature. This dephlogisticated air, however, is but a small part of the composition of our atmosphere. According to the most accurate computations, the air we usually breathe is composed of only one-fourth of this dephlogisticated air, or perhaps less; the other three or four parts consisting of what Dr Priestley calls phlogisticated, and M. Lavoisier mephitic air. This by itself is absolutely noxious, and exceedingly poisonous to animals; though it seems only to be negatively so; for when mixed in a certain proportion with dephlogisticated air, it may be breathed with safety, which could not be if it contained any ingredient absolutely unfriendly to the human constitution.

The other part, viz., the pure dephlogisticated air, seems to stand much in the same relation to plants that phlogisticated air does to animals; that is, it would prove poisonous and destroy them if they were to depend upon it entirely for their subsistence; but as they derive their nourishment partly from the air and partly from the soil, it thence happens, that the plants which are set to grow in dephlogisticated air do not die instantly, as animals do in the phlogisticated kind, but remain for some time weak and sickly.

The other component parts of our atmosphere are so various, and of such heterogeneous natures, that they do not admit of any kind of definition or analysis, one only excepted, namely, the electric fluid. This we know pervades the whole, but appears to be much more copious in the upper than in the lower atmospherical regions. See Electricity. To measure the absolute quantity of this fluid, either in the atmosphere or any other substance, is impossible. All that we can know on this subject is, that the electric fluid pervades the atmosphere; that it appears to be more abundant in the superior than the inferior regions; that it seems to be the immediate bond of connection between the atmosphere and the water which is suspended in it; and that, by its various operations, the phenomena of hail, rain, snow, lightning, and various other kinds of meteors, are occasioned.

Various attempts have been made to ascertain the height to which the atmosphere is extended all round the earth. These commenced soon after it was discovered, by means of the Torricellian tube, that air is a gravitating substance. Thus it also became known, that a column of air, whose base is a square inch, and the height that of the whole atmosphere, weighs 15 pounds; and that the weight of air is to that of mercury, as 1 to 10,800; whence it follows, that if the weight of the atmosphere be sufficient to raise a column of mercury to the height of 30 inches, the height of the aerial column must be 10,800 times as much, and consequently a little more than five miles high.

It was not, however, at any time supposed, that this calculation could be just; for as the air is an elastic fluid, the upper parts must expand to an immense bulk, and thus render the calculation above related exceedingly erroneous. By experiments made in different countries, it has been found, that the spaces which any portion of air takes up, are reciprocally proportional to the weights with which it is compressed. Allowances were therefore to be made in calculating the height of the atmosphere. If we suppose the height of the whole divided into innumerable equal parts, the density of each of which is as its quantity; and the weight of the whole incumbent atmosphere being also as its quantity; it is evident, that the weight of the incumbent air is everywhere as the quantity contained in the subjacent part; which makes a difference between the weights of each two contiguous parts of air. By a theorem in geometry, where the differences of magnitudes are geometrically proportional to the magnitudes themselves, these magnitudes are in continual arithmetical proportion; therefore, if, according to the supposition, the altitude of the air, by the addition of new parts into which it is divided, do continually increase in arithmetical proportion, its density will be diminished, or (which is the same thing) its gravity decreased, in continual geometrical proportion.

It is now easy, from such a series, by making two or three barometrical observations, and determining the density of the atmosphere at two or three different stations, to determine its absolute height, or its rarity, at any assignable height. Calculations accordingly were made upon this plan; but it having been found that the barometrical observations by no means corresponded with the density which, by other experiments, the air ought to have had, it was suspected that the upper parts of the atmospherical regions were not subject to the same laws with the lower ones. Philosophers therefore had recourse to another method for determining the altitude it determined from which the light of the sun is refracted, so as to begin to become visible to us before he himself is seen in the heavens. By this method it was determined, that at the height of 45 miles the atmosphere had no power of refraction; and consequently beyond that distance was either a mere vacuum or the next thing to it, and not to be regarded.

This theory soon became very generally received, and the height of the atmosphere was spoken of as familiarly as the height of a mountain, and reckoned to be as well ascertained, if not more so, than the heights of most mountains are. Very great objections, however, which have never yet been removed, arise from the appearances of some meteors, like large globes of appearance fire, not frequently to be seen at vast heights above the earth (see Meteor). A very remarkable one of this kind was observed by Dr Halley in the month of March 1719, whose altitude he computed to have been between 69 and 73½ English miles; its diameter 2800 yards, or upwards of a mile and a half; and its velocity about 350 miles in a minute. Others, apparently of the same kind, but whose altitude and velocity were still greater, have been observed; particularly that very remarkable one, August 18th, 1783, whose distance from the earth could not be less than 95 miles, and its diameter not less than the former; at the same time that its velocity was certainly not less than 1000 miles in a minute. Fire-balls, in appearance similar to these, though vastly inferior in size, have been sometimes observed at the surface of the earth. Of this kind Dr Priestley mentions one seen on board the Montague, 4th November 1749, which appeared as big as a large millstone, and broke with a violent explosion.

From analogical reasoning, it seems very probable, that the meteors which appear at such great heights in the air are not essentially different from those which, like the fire-ball just mentioned, are met with on the surface of the earth. The perplexing circumstances with regard to the former are, that at the great heights above mentioned, the atmosphere ought not to have any density sufficient to support flame, or to propagate sound; yet these meteors are commonly succeeded by one or more explosions, nay are sometimes said to be accompanied with a hissing noise as they pass over our heads. The meteor of 1719 was not only very bright, insomuch that for a short space it turned night into day, but was attended with an explosion heard over all the island of Britain, occasioning a violent concussion in the atmosphere, and seeming to shake the earth itself. That of 1783 also, though much higher than the former, was succeeded by explosions; and, according to the testimony of several people, a hissing noise was heard as it passed. Dr Halley acknowledged that he was unable to reconcile these circumstances with the received theory of the height of the atmosphere; as, in the regions in which this meteor moved, the air ought to have been 300,000 times more rare than what we breathe, and the next thing to a perfect vacuum.

In the meteor of 1783, the difficulty is still greater, as it appears to have been 20 miles farther up in the air. Dr Halley offers a conjecture, indeed, that the vast magnitude of such bodies might compensate for the thinness of the medium in which they moved. Whether or not this was the case indeed cannot be ascertained, as we have so few data to go upon; but the greatest difficulty is to account for the brightness of the light. Appearances of this kind are indeed with great probability attributed to electricity, but the difficulty is not thus removed. Though the electrical fire pervades with great ease the vacuum of a common air-pump, yet it does not in that case appear in bright well-defined sparks, as in the open air, but rather in long streams resembling the aurora borealis. From some late experiments, indeed, Mr Morgan concludes, that the electrical fluid cannot penetrate a perfect vacuum*. If this is the case, it shows that the regions we speak of are not such a perfect vacuum as can be artificially made; but whether it is or not, the extreme brightness of the light shows that a fluid was present in those regions, capable of confining and condensing the electric matter as much as the air does at the surface of the ground; for the brightness of these meteors, considering their distance, cannot be supposed inferior to that of the brightest flashes of lightning.

This being the case, it appears reasonable to conclude, that what is called the density of the air does not altogether keep pace with its gravity. The latter indeed must in a great measure be affected by the vapours, but above all by the quantity of the basis of fixed or deplogisticated air contained in it: for Mr Kirwan has discovered that the basis of fixed air, when deprived of its elastic principle, is not greatly inferior to gold in specific gravity; and we cannot suppose that of deplogisticated air to be much less. It is possible, therefore, that pure air, could it be deprived of all the water it contains, might have very little gravity; and as there is great reason to believe that the basis of deplogisticated air is only one of the constituent parts of water, we see an evident reason why the air ought to become lighter, and likewise less fit for respiration, the higher up we go, though there is a possibility that its density, or power of supporting flame, may continue unaltered.

There are not yet, however, a sufficient number of facts to enable us to determine this question; though such as have been discovered seem rather to favour the above conjecture. Dr Boerhaave was of opinion that the gravity of the air depended entirely on the water it contained; and, by the means of alkaline salts, he was enabled to extract as much water from a quantity of air as was very nearly equivalent to its weight. By the calculation of metals we may extract as much of the basis of deplogisticated air from a quantity of atmospheric air as is equivalent to the weight of air lost. Were it possible, therefore, to extract the whole of this, as well as all other vapours, and to preserve only the elastic principle, it is highly probable that its gravity would entirely cease. It has been found, by those who have ascended with aerostatic machines, or to the tops of high mountains, that the deplogisticated air is found to be contained in smaller quantities in the atmosphere of those elevated regions than on the lower grounds. It is also found, that in such situations the air is much drier, and parts with water with much more difficulty, than on the ordinary surface. Salt of tartar, for instance, which at the foot of a mountain will very soon run into a liquid, remains for a long time exposed to the air on the top of it, without showing the least tendency to deliquesce. Nevertheless, it hath never been observed that fires did not burn as intensely on the tops of the highest mountains as on the plains. The matter indeed was put to the trial in the great eruption of Vesuvius in 1779, where, though the lava spouted up to the height of three miles above the level of the sea, the uppermost parts all the while were to appearance as much inflamed as the lowest.

The high degree of electricity, always existing in the upper regions of the atmosphere, must of necessity have a very considerable influence on the gravity of any heterogeneous particles floating in it. When we consider the effects of the electric fluid upon light bodies kept at the surface of the earth, it will readily be admitted, nigh it that in those regions where this fluid is very abundant, the gravity of the atmosphere may be much diminished without affecting its density. We know that it is the nature of any electrified substance to attract light bodies; and that, by proper management, they may even be suspended in the air, without either moving up or down for a considerable time. If this is the case with light terrestrial bodies, it cannot be thought very improbable that the aerial particles themselves, i.e., those which we call the basis of deplogisticated air, and of aqueous or other vapour diffused among them, should be thus affected in the regions where electricity is so abundant. From this cause, therefore, also the gravity of the atmosphere may be affected without any alteration at all being made in its density; and hence may arise anomalies in the barometer hitherto not taken notice of.

It appears, therefore, that the absolute height of the Absolute atmosphere is not yet determined. The beginning height of and ending of twilight indeed show, that the height at which the atmosphere begins to refract the sun's light is about 44 or 45 English miles. But this may not improbably be only the height to which the aqueous vapours are carried; for it cannot be thought any unreasonable supposition, that light is refracted only by means of the aqueous vapour contained in the atmosphere; and that where this ceases, it is still capable of supporting the electric fire at least as bright and strong as at the surface. That it does extend much higher, is evident from the meteors already mentioned; for all these are undoubtedly carried along with the atmosphere;