2. The composition of that part of our atmosphere properly called air, was till lately but very little known. Formerly, it was supposed to be a simple, homogeneous, and elementary fluid. But the discoveries of late philosophers have confuted this opinion. Dr. Black, in his observations on the nature of heat or elementary fire, discovered that on being admitted into solid bodies in a larger quantity than usual, it expanded them; that the next effect was the reducing them to a state of fluidity; and lastly, they were converted into an elastic fluid. Hence it is inferred that heat or caloric is the only permanent natural fluid, and the cause of fluidity in other substances. But some of these aëriform fluids continue elastic only in a high temperature; as vapours from water, smoke, &c. while others retain the aërial form in every state of the atmosphere. This has produced a distinction between elastic fluids, and permanently elastic fluids, which last are called gases, from gas or spirit. Atmospheric air is composed of two elementary substances: (1.) oxygen, in the proportion of about one-fourth, and, (2.) nitrogen gas or uzote, about three-fourths suspended in a mass of caloric. It contains also nearly one part in every thousand of carbonic acid gas and several adventitious substances. This air supports combustion and animal life by the agency of the oxygen gas, and, as this gas is diminished or taken away, life is suspended or destroyed. See Chap. XII. CHEMISTRY.

3. Though the air seems to be a repository for all the poisonous effluvia arising from putrid and corrupted substances; yet it has a wonderful facility of purifying itself, and of depositing those vapours, so that it never becomes noxious except in particular places, and for a short time. The air is generally purified by water; the quantity of this aqueous vapour contained in the atmosphere is immense. From an experiment made on the evaporation from a fluid surface heated to the same degree with that given by our meridian sun, it has been calculated, that the evaporation from the Mediterranean Sea in a summer's day is 5280 millions of tuns of water, which is more than it receives from all the nine large rivers that empty themselves into it. From some experiments made with a view to determine the quantity of water raised from the earth alone in time of drought, it has been ascertained, that when there

had been no rain for above a month, and the grass was become quite brown and parched, the evaporation from an acre was not less than 1600 gallons in twenty-four hours. By two other experiments also, it was found, when the ground had been moistened by rain the day before, the one gave 1973, the other 1905 gallons, in twelve hours. From this, the air is every moment purified by the ascent of the vapour, which, flying off into the clouds, thus leaves a space for the exhalation of fresh quantities. The vapour being considerably lighter than the common atmosphere, and in consequence, ascending with great velocity, the air during all this time is said to be dry, notwithstanding the vast quantity of aqueous fluid that passes through it.

4. Plants derive subsistence from the azote or air that is unfit for respiration, and in return, give out the oxygen or vital air, upon the enjoyment of which life depends. The plant purifies what the animal had poisoned; in return, the contaminated air is more than ordinarily nutritious to the plant. Agitation with water is another restorative. The foulest air shaken in a bottle with water for a sufficient length of time, recovers a great degree of its purity. Hence, we see the salutary effects of storms and tempests. If the atmosphere were every where equally dense, it would not be much more than five miles in height. But the air being very elastic, and the more it is pressed, the less space it occupies, it follows that in the upper regions of the atmosphere it must become rarer as it ascends; hence, the height of the atmosphere is found to be about forty-five miles.

5. The Barometer is an instrument for measuring the weight of the atmosphere, in order chiefly to determine the changes of weather. The common barometer is a glass tube about two-tenths of an inch in diameter, and its length at least thirty-one inches. This tube is filled with mercury so as not to have any air over it, the maker placing his finger on the end, immerses it in a bason of quicksilver, and then takes his finger away. The quicksilver in the tabe by its own weight endeavours to descend into that of the bason: but the external air pressing on the surface of the quicksilver in the bason without, and no air being in the tube at top, the quicksilver will continue

in the tube, being raised by the air on the surface in the bason below. The usual range of the barometer in this country is from twenty-eight to thirty-one inches; when the air is pure and heavy, it raises the mercury to nearly thirty-one, and when light and full of vapours, it falls to nearly twenty-eight. In fine dry weather, the air is rendered pure, free from all light vapours, and is consequently. extremely heavy, so that it presses up the quicksilver. In moist rainy weather the atmosphere being charged with vapours, clouds, and fogs, the air is then sensibly lighter, and presses upon the quicksilver with less force. When high winds blow, the atmosphere is light, and the quicksilver generally is low, and it rises higher in cold weather than in warm. During frost, the air is purest and heaviest, and the barometer rises to its highest points. This instrument is also serviceable in measuring the height of nountains. In ascending mountains, quicksilver is found to sink about a tenth of an inch in ninety feet; so that if the quicksilver fall an inch, we have ascended near mine hundred feet; but this is subject to variations, from change of temperature and other causes, which render various corrections necessary. The general method, however, of determining altitudes by the barometer and thermometer is extremely useful and convenient; and ingenious rules are given by Dr. Hutton, Dr. Gregory, Sir Henry Englefield, and others, to facilitate the computation.

6. The Thermometer shows the variations in the temperature of the weather. It is constructed with a hollow ball of glass, and a long tube partly filled with mercury or spirits of wine, tinctured, so as to be seen when it rises. The ball is plunged into boiling water, which causes the spirit to expand to its greatest height, and at this point the tube is broken off, and hermetically sealed; that is, the neck of the tube being heated, till it is just ready to melt, it is twisted together with a pair of hot pincers. A scale being added completes the thermometer. All bodies expanding by heat and contracting by cold, the relative temperature is easily ascertained. The thermometer most used in England is that of Fahrenheit; though the centigrade thermometer first invented in Sweden and now generally employed on the continent, is, in many respects, far preferable. The hygrometer is an

instrument to measure the degrees of dryness or moisture of the atmosphere. A simple hygrometer may be made by using a flaxen line five feet long; and having a graduated scale fixed to an index moving on a fulcrum. The length of the index, from the fulcrum to the point, should be ten inches; that of the lever, from the fulcrum to the middle of the eye, to which the cord is fixed, two and a .half. The air becoming moist, the cord imbibes its moisture; the line, in consequence, is shortened, and the index rises. On the contrary, the air becoming dry, the cord discharges its moisture,-lengthens,-and the index falls. By the pluviometer or rain-gauge inay be measured the quantity of rain that has fallen. It consists of a funnel, twelve inches in diameter attached to a tube four inches in diameter, and in the tube is a floating graduated index, which rises as the rain falls in the funnel and tube.

7. Clouds, are collections of vapours that float in the atmosphere; having different degrees of opacity, which arise from their extent and density. Their height above the surface of the earth (not above the mountains) is various, - but hardly ever exceeds a mile, or a mile aud a half. In hot weather, or hot climates, the clouds, being more rarefied, are lighter, and ascend much higher than they do in colder climates, or colder weather; and, indeed, in cold weather, the clouds frequently touch the very surface of the earth; for a fog is nothing but a cloud close to the ground. A mist is an incipient formation of clouds, or haziness; and often denotes a very small rain, or a deposition of water in particles so small as not to be visible singly. Snow is formed when the atmosphere is so cold as to freeze the particles of rain as soon as they are formed, and the adhesion of several of these particles to each other, during their descent through the air, forms the usual fleeces of snow, which are larger, when the clouds are higher. Hail differs from snow in its consisting of much more solid, and much more defined pieces of congealed water. The water, already formed into considerable drops, is driven through a cold region of the atmosphere, by the wind, which almost always accompanies a fall of hail. The hail-stones, or globules of ice, sometimes far exceed the usual size of drops of rain; this shows, that, by the action of the wind, the congealed particles are compelled to adhere to each

other. Dew and hour-frost, seem to proceed from a quantity of aqueous and undecomposed vapour, which always exists in the atmosphere; and which, being raised by heat, is condensed by cold, without undergoing that process by which water is changed into air. If the cold be very intense, hoar-frost appears instead of dew; which is nothing more than the dew frozen after it falls upon the ground, in the same manner that the vapour, in a warm room, congeals on the inside of the windows, in a frosty night.

8. Wind. When the air is rarefied, it naturally ascends into the higher regions; and the circumjacent air, which is thicker and heavier, immediately rushes in to supply its place, and fill up the vacancy. This motion of the air we call wind, and it is named a breeze, a gale, or storm, according to the velocity of its motion. The tropical or general trade winds, called tropical winds, which blow almost always from the same point of the compass, extend to near thirty degrees of lati tude on each side the equator, in the Atlantic, Ethiopie, and Pacific Oceans. On the north side of the equator they blow from north-east, on the south side from south-east, and near the equator from almost due east. The monsoons, or shifting trade winds, blow six months in one direction, and the other six months in an opposite direction. These are mostly in the Indian or Eastern Ocean, and do not reach above two hundred leagues from the land. Their change is at the vernal and autumnal equinox, and is accompanied with terrible storms of thunder, lightning, and rain. The land and sea breezes, which are periodical winds, blow from the land from night to about mid-day, and from the sea from about noon to midnight. These winds do not extend above two or three leagues from the shore.

9. Hurricanes are those sudden and violent gusts of wind which come on at very irregular periods, and generally continue for a short time. They sometimes spread over an extensive tract of country, and at others, are confined within a remarkably narrow space. Tornadoes are violent winds attended with particular phenomena; such as droughts, or heavy rains, hail, snow, or thunder and lightning, or several of those phenomena at