tion. The first of these are the only true Atheists, and it has been doubted whether such persons really exist; yet it must be confessed, that in the year 1619, Spinosa was burnt to death for having avowed his adherence to the opinion. We have many excellent works in opposition to Atheism, but not a single treatise written in its behalf. Those who wish to see Atheism confuted, may be referred to the sermons preached at Boyle's Lectures; to Abernethy's Discourses on the Attributes; and above all to Paley's Natural Theology. Newton, Boyle, Maclaurin, and others, among the most distinguished mathematicians and philosophers, have been among the ablest advocates for the existence of a God.

ATHENÆA, in botany, a genus of plants of the Octandria Monogynia class and order. Essential character: calyx coloured, five-parted; no corolla ; bristles eight-feathered, between the filaments; stigma five-parted; capsule globose, onecelled, three-valved; seeds three to five. There is one species, a branching shrub; stem four or five inches in diameter, covered with a wrinkled grey bark. The flowers came out in bundles from the axils; their calyx is white; capsule green, with a tinge of violet. The seeds are covered with a pulpy viscid membrane, of a scarlet colour; the bark, leaves, and fruits are sharp and aromatic. The last are called by the Creoles caffe diable. It is a native of Cayenne and the neighbouring continent of Guiana, a mile from the shore, in a sandy soil, flowering and bearing fruit in September.

ATHENÆUM, in antiquity, a public place wherein the professors of the liberal arts held their assemblies, the rhetoricians declaimed, and the poets rehearsed their performances.

ATHERINA, in natural history, a genus of fishes of the order Abdominales. Head somewhat flattened over the upper-jaw; gill-membrane six-rayed; body marked by a silver lateral stripe. There are five species: A hespetus has an aral fin with about twelve rays; it inhabits the Mediterranean, European, and Red seas; about three or four inches long; body varied with a few black spots, and nearly pellucid. This species, which is named Athernos by the modern Greeks, is seen in vast shoals about the coasts of the islands in the Archipelago, and is easily taken in great quantities by the simple device of trailing in the water a horse's tail or a piece of black-cloth fastened to the end of a pole, the fishes following all its motions, and suffering themselves to be

drawn into some deep cavity formed by the rocks, when they are readily secured by means of a net, and may be taken at pleasure. At Southampton they are to be had at almost all seasons, where they go by the name of smelts. See Plate Pisces, fig. 4. ATHWAŔT, in naval affairs, across the line of the ship's course, as "We discovered a fleet standing athwart us," i. e. steering across our way.

ATIWART hawse, the situation of a ship when she is driven by any accident across the stem of another, whether they bear against, or at a small distance from each other: the transverse position being principally understood.

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 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 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 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 is that invisible elastic fluid which surrounds the earth to an unknown height, and encloses it on all sides. This fluid is essential to the existence of all animal and vegetable life, and even to the constitution of all kinds of matter whatever, without which they would not be what they are: for by it we literally may be said to live, move, and have our being: by insinuating itself into all the pores of bodies, it becomes the great spring of almost all the mutations to which the chemist and philosopher are witnesses in the changes of bodies. Without the atmosphere no animal could exist; vegetation would cease, and there would be neither rain nor refreshing dews to moisten the face of the ground; and though the sun and stars might be seen as bright specks, yet there would be little enjoyment of light,

could we ourselves exist without it. Nature indeed, and the constitutions and principles of matter, would be totally changed if this flaid were wanting.

The mechanical force of the atmosphere is of great importance in the afairs of men, who employ it in the motion of their ships, in turning their mills, and in a thousand other ways connected with the arts of life. It was not till the time of Lord Bacon, who taught his countrymen how to investigate natural phenomena, that the atmosphere began to be investigated with any degree of precision. Galileo introduced the study by pointing out its weight; a subject that was soon after investigated more completely by Torricelli and others. Its density and elasticity were ascertained by Mr. Boyle and the academicians at Florence. Mariotte measured its dilatibility; Hooke, Newton, Boyle, and Derham, shewed its relation to light, to sound, and to electricity. Sir I. Newton explained the effect produced upon it by moisture, from which Halley attempted to explain the changes in its weight indicated by the barometer.

The atmosphere, we have said, envelops the whole surface of the earth, and if they were both at rest, then the figure of the atmosphere would be globular, because all the parts of the surface of a fluid in a state of rest must be equally removed from its centre. But as the earth and the surrounding parts of the atmosphere revolve uniformly together about their axis, the different parts of both have a centrifugal force, the tendency of which is more considerable, and that of the centripetal less, as the parts are more remote from the axis, and hence the figure of the atmosphere must become an oblate spheroid, since the parts that correspond to the equator are farther removed from the axis than the parts which correspond to the poles. The figure of the atmosphere must also, on another account, represent a flattened spheroid, namely, because the sun strikes more directly the air which encompasses the equator, and is comprehended between the two tropics, than that which pertains to the polar regions: hence it follows, that the mass of air, or part of the atmosphere adjoining to the poles, being less heated, cannot expand so much nor reach so high. Nevertheless, as the same force which contributes to elevate the air diminishes its gravity and pressure on the surface of the earth, higher columns of it about the equatorial parts, other eircumstances being the same, may not be heavier than those about the poles. Mr. Kirwan observes, that in

the natural state of the atmosphere, that is, when the barometer would, every where at the level of the sea, stand at 30 inches, the weight of the atmosphere at the surface of the sea must be equal all over the globe; and in order to produce this equality, as the weight proceeds from its density and height, it must be lowest where the density is greatest, and highest where the density is least, that is, highest at the equator and lowest at the poles, with the intermediate gradations. On this and other accounts, in the highest regions of the atmosphere, the denser equatorial air not being supported by the collateral tropical columns, gradually flows over and rolls down to the north: and south; these superior tides have been supposed to consist of hydrogen gas, inasmuch as it is much lighter than any other, and is generated in great plenty between the tropics; it is also supposed to furnish the matter of the auroræ borealis and australis.

With regard to the weight and pressure of the atmosphere, it is evident that the whole mass, in common with all other matter, must be endowed with weight and pressure: and it is found by undeniable experiments, that the pressure of the atmosphere sustains a column of quicksilver in the tube of a barometer of about 30 inches in height; it accordingly follows, that the whole pressure of the atmosphere is equal to the weight of a column of quicksilver of an equal base, and 30 inches in height, or the weight of the atmosphere on every square inch of surface is equal to 15 pounds. It has moreover been found, that the pressure of the atmosphere balances, in the case of pumps, &c. a column of water 34 feet high; and the cubical foot of water weighing just 1000 ounces, or 621 pounds, 344 multiplyed by 621, or 2158 lb. will be the weight of a column of water, or of the atmosphere on the base of a square foot; and consequently the 144th part of this, or 15 lb. is the weight of the atmosphere on a square inch. From these data, Mr. Cotes computed the pressure of the atmos phere on the whole surface of the earth to be equivalent to that of a globe of lead 60 miles in diameter. Dr. Vince and others have given the weight at 77670297973563429 tons. This weight is however variable; it sometimes being much greater than at others. If the surface of a man, for instance, be equal to 14 square feet, the pressure upon him, when the atmosphere is in its lightest state, is equal to 13 tons, and when in the heaviest, it is about 14 tons and one-third"; the difference of which is about 2464 lb. It is surprizing that such weights should be able

to be borne without crushing the human frame: this indeed must be the case, if all the parts of our body were not endowed with some elastic spring, whether of air or other fluid, sufficient to counterbalance the weight of the atmosphere, Whatever this spring is, it is certain that it is just able to counteract the weight of the atmosphere, and no more; of course it must alter in its force as the density of the atmosphere varies: for if any considerable pressure be superad. ded to that of the air, as by going into deep water, it is always severely felt; and if, on the other hand, the pressure of the atmosphere be taken off from any part of the human body, by means of the apparatus belonging to the air pump, the inconvenience is immediately perceived.

The difference in the weight of the atmosphere is very considerable, as has been observed, from the natural changes in the state of the air. These changes take place chiefly in countries at a distance from the equator. In Great Britain, for instance, the barometer varies from 28.4 to 30.7. On the increase of this natural weight, the weather is commonly clear and fine, and we feel ourselves alert and active; but when the weight of the air diminishes, the weather is often bad, and we feel listlessness and inactivity. Hence invalids suffer in their health from very sudden changes in the atmosphere. In our observations on the barometer, we have known the mercury to vary a full inch, or even something more, in the course of a few hours. Such changes, however, are by no means frequent. Ascending to the tops of mountains, where the pressure of the air is very much diminished, the inconvenience is rarely felt, on account of the gradual change; but when a person ascends in a balloon with great rapidity, he feels, we are told by Garnerin and other aeronauts, a difficulty of breathing, and many unpleasant sensations. So also, on the condensation of the air, we feel little or no alteration in ourselves, except when the variations are sudden in the state of the atmosphere, or by those who descend to great depths in a diving-bell, See DIVING-Bell.

It is not easy to assign the true reason for the changes that happen in the gravity of the atmosphere in the same place. One cause is, undoubtedly, the heat of the sun; for where this is uniform, the changes are small and regular. Thus, between the tropics the barometer constantly sinks about half an inch every day, and rises to its for

mer station in the night. But in the temperate zones, the altitude of the mercury is subject to much more considerable variations, as we have seen with respect to what is observable in our own country.

As to the alteration of heat and cold, Dr. Darwin infers, that there is good reason to conclude that in all circumstances where air is mechanically expanded, it becomes capable of attracting the fluid matter of heat from other bodies in contact with it. Now, as the vast region of air which surrounds our globe is perpetually moving along its surface, climbing up the sides of mountains, and descending into the valleys ; as it passes along it must be perpetually varying the degree of heat according to the elevation of the country it traverses: for, in rising to the summits of mountains, it becomes expanded, having so much of the pressure of the superincumbent atmosphere taken away; and when thus expanded, it attracts or absorbs heat from the mountains in contiguity with it; and, when it descends into the valleys and is compressed into less compass, it again gives out the heat it has acquired to the bodies it comes in contact with. The same thing must happen in the higher regions of the atmosphere, which are regions of perpetual frost, as has lately been* discovered by the aerial navigators. When large districts of air, from the lower parts of the atmosphere, are raised two or three miles high, they become so much expanded by the great diminution of the pressure over them, and thence become so cold, that hail or snow is produced by the precipitation of the vapour: and as there is, in these high regions of the atmosphere, nothing else for the expanded air to acquire heat from after it has parted with its vapour, the same degree of cold continues till the air, on descending to the earth, acquires its former state of condensation and of warmth. The Andes, almost under the line, rests its base on burning sands: about its middle height is a most pleasant and temperate climate covering an extensive plain, on which is built the city of Quito; while its forehead is encircled with eternal snow, perhaps coeval with the mountain. Yet, according to the accounts of Don Ulloa, these three discordant climates seldom encroach much on each other's territories. The hot winds below, if they ascend, become cooled by their expansion; and hence they cannot affect the snow upon the summit; and the cold winds that sweep the summit, become

condensed as they descend, and of temperate warmth before they reach the fertile plains of Quito.

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 ineans of the Torricellian tube, that air is endued with weight and pressure. And had not the air an elastic power, but were it every where of the same density, from the surface of the earth to the extreme limit of the atmosphere, like water, which is equally dense at all depths, it would be a very easy matter to determine its height from its density and the column of mercury which it would counterbalance in the barometer tube: for, it having been observed that the weight of the atmosphere is equivalent to a column of 30 inches or 2 feet of quicksilver, and the density of the former to that of the latter, as 1 to 11040; therefore the height of the uniform atmosphere would be 11040 times 24 feet, that is 27600 feet, or little more than 5 miles and a quarter. But the air, by its elastic quality, expands and contracts; and it being found by repeated experiments in most nations of Europe, that the spaces it occupies, when compressed by different weights, are reciprocally proportional to those weights themselves; or, that the more the air is pressed, so much the less space it takes up; it follows that the air in the upper regions of the atmosphere must grow continually more and more rare, as it ascends higher; and indeed that, according to that law, it must necessarily be extended to an indefinite height. Now, if we suppose the height of the whole divided into innumerable equal parts; the quantity of each part will be as its density; and the weight of the whole incumbent atmosphere being also as its density; it follows, that the weight of the incumbent air is every where as the quantity contained in the subjacent part; which causes a difference between the weights of each two contiguous parts of air. But, by a theorem in arithmetic, when a magnitude is continually diminished by the like part of itself, and the remainders the same, these will be a series of continued quantities decreasing in geometrical progression: 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 progression, its density will be diminished, or, which is the same thing, its gravity decreased, in con

tinued geometrical proportion. And hence, again, it appears that, according to the bypothesis of the density being always proportional to the compressing force, the height of the atmosphere must necessarily be extended indefinitely. And, farther, as an arithmetical series adapted to a geometrical one, is analogous to the logarithms of the said geometrical one; it follows therefore that the altitudes are proportional to the logarithms of the densities, or weights of air; and that any height taken from the earth's surface, which is the difference of two altitudes to the top of the atmosphere, is proportional to the difference of the logarithms of the two densities there, or to the logarithm of the ratio of those densities, or their corresponding compressing forces, as measured by the two heights of the barometer there.

It is now easy, from the foregoing property, and two or three experiments, or barometrical observations, made at known altitudes, to deduce a general rule to determine the absolute height answering to any density, or the density answering to any given altitude above the earth. And accordingly, calculations were made upon this plan by many philosophers, particularly by the French; but it having been found that the barometrical observations did not correspond with the altitudes as measured in a geometrical manner, it was suspected that the upper parts of the atmospherical regions were not subject to the same laws with the lower ones, in regard to the density and elasticity. And indeed, when it is considered that the atmosphere is a heterogeneous mass of particles of all sorts of matter, some elastic, and others not, it is not improbable but this may be the case, at least in the regions very high in the atmosphere, which it is likely may more copiously abound with the electrical fluid. Be this however as it may, it has been discovered that the law above given, holds very well for all such altitudes as are within our reach, or as far as to the tops of the highest mountains on the earth, when a correction is made for the difference of the heat or temperature of the air only, as was fully evinced by M. De Luc, in a long series of observations, in which he determined the altitudes of hills both by the barometer, and by geometrical measurement, from which he deduced a practical rule to allow for the difference of temperature. Similar rules have also been deduced from accurate experiments, by Sir George Shuck