the air upon a square inch of the earth's surface is fifteen pounds; when it falls so low as to twenty-eight inches, the weight on a square inch is not quite fourteen one-fourth pounds, and when it rises to thirty-one inches the air will press on each square inch with a weight of fifteen pounds three quarters. Supposing the surface of the body of a middle sized man to measure fourteen square feet, or 2016 square inches, he sustains a pressure, (at the medium weight of air of fifteen pounds on each square inch,) of 30,240 lb. Troy, or eleven tons two cwt. eighteen onehalf lb. But the air pressing equally on all sides, within the body as well as without, no action is produced upon the fibres, so as to derange them, but rather to brace them together, and keep each in its proper place, consequently no sensation of pressure or weight can be excited. The fish is no more exposed to be crushed by the weight of the water above when lying in the depths of the sea, than when within a few inches of the surface. In the surgical operation called cupping, a num ber of small shallow openings are made in the skin, but little or no blood follows the wounds. A cup or small vessel in which some tow or light combustible substance is set on fire, is inverted over the place. The fire consumes the air in the cup, which is then closely applied to the skin; and the pressure of the air being thus taken off from that part, while it acts with full force on all the other parts of the body, the blood is forced out through the small punctures made by the instrument.

As all the parts of the atmosphere gravitate or press upon each other, we can easily conceive why the air should be denser or more compressed near to them at a distance from the earth. Let a quantity of fine light wool be thrown gently into a pit until it be filled. The wool in the bottom, having the weight of the whole quantity to support, will be compressed into less thickness than an equal quantity lying in the middle of the depth of the pit, while that near the top will occupy nearly as much room as the light parcels as first dropped in. The same is the case with our air or the atmosphere: consequently, if we were to make experiments on the weight of the air, taken at regular equal distances upwards from the surface of the earth, this weight would be found to diminish according to a stated proportion. This proportion has been computed to be such that if the heights from the earth be taken in arithmetical proportion, the rarity of the air will be in geometrical proportion: or, in other words, if the height be doubled, the rarity will be as the square of the first rarity. If the height be tripled, the rarity will be as the cube, and so on. Gravity or weight being in proportion to the number of particles of matter, contained within a given space, as these particles are placed nearer to, or farther from, each other, the body they compose is said to be more or less dense or thick, and less or more rare or thin. The thinness or rarity of the air increasing as we rise in it, agreeably to the foregoing proportion, it follows that whatever be the rarity of the air on the surface of the earth, at a height of 3 miles the rarity of the air will be doubled, or its weight will be reduced to one half: at seven miles of height, the rarity will be four times greater, or the weight will be reduced to one-fourth of that on the earth, &c. This fact is not known from theory only, for it has been confirmed by experiments made on the most elevated mountains in the world; as on the summit of Mont Blanc in the Alps, which rises very nearly three English miles above the level of the sea.

The Barometer. The column of mercury supported by the pressure of the atmosphere in a tube devoid of air is of different heights according to that pressure, or to the weight of the external air; hence the machine obtained the name of barometer, from Greek words signifying the weight-measurer. It was also observed, that variations in the length of the column of mercury, seemed to be generally accompanied by variations in the state of the weather: hence the machine was called the weatherglass. The barometer consists of a straight glass tube or pipe a quarter or rather one-third of an inch in diameter, and thirty-four inches long. It is hermetically sealed at one end (that is, the end is closed by melting the sides together) and made perfectly clean and dry. Ă quantity of the purest mercury is poured into the tube, and by placing a finger on the open end, when it is full, the tube is turned downwards, and sunk below the surface of mercury placed in a small bason. The finger being now removed, the mercury in the tube will sink down till it be in weight equal to that of an equal column of air pressing on the surface of the mercury in the bason. By this sinking, a space of four or five inches or so, will be left in the upper part of the tube, above the mercury, entirely free from air, and therefore a perfect vacuum is produced, in which the fluid may rise or fall without obstruction. The tube and bason are then attached to a frame, on the upper part of which is a portion of a scale of inches and tenths, accurately measured from the surface of the mercury in the bason.

The Thermometer. As the mercury may be influenced in its rise and fall, by the heat or cold, as well as by the weight of the atmosphere; another instrument should be attached to the barometer, to indicate the variations of temperature in respect to heat and cold. This instrument, called a thermometer, from Greek terms signifying a heat-measurer, resembles the barometer in being a glass tube of a small bore, blown up at one end into a ball or bulb. The bulb and a part of the tube being filled with mercury, the other end is melted together. Mercury is employed in this instrument, because it expands or enlarges its bulk when exposed to heat, more equably than any other fluid. Of this instrument various kinds are in use; but that employed in this country is Fahrenheit's, so called from the inventor of its scale. In measuring the degrees of heat, the standard points are those of freezing and boiling water, which are constantly the same in all parts of the world. The point of the tube at which the upper surface of the mercury stands, when immersed in freezing water, is marked on the scale thirty-two parts or degrees, and observing to what height the mercury rises, when immersed in boiling water, the distance between the two is divided into 180 equal parts, which added to thirty-two, give 212 for the height at the heat of boiling water in the open air. Water cannot be made colder in the open air than thirty-two parts or degrees, because, at that temperature it ceases to be a fluid, being converted into ice; nor hotter than 212 degrees, because at that temperature it loses the form of water, and is converted into vapour or steam. The reason why the temperature of freezing water was fixed at thirty-two degrees, was, that Fahrenheit found the most intense cold he could produce was by mixing snow and salt together. Placing the thermometer in this mixture, the point to which the mercury sunk, he marked O, as the beginning of his sclale; and the space between this point and that at which the mercury stood, when placed in freezing water, was found

to contain thirty-two of those equal parts, of which 180 filled up the space between the points of freezing and boiling water. But much more intense cold can now be produced, than was formerly imagined by the mixture of substances which dissolve rapidly; such are neutral salts dissolved in water, diluted acids and some neutral salts, snow or pounded ice, and some salts; and a number of equal parts or degrees being reckoned downwards on the scale from O or zero, the degress of cold may be measured as those of heat upon the upper part of the scale. A mixture of eight parts of sulphat of soda (formerly called Glauber's salts) with five parts of muriatic acid (or acid of sea-salt) will sink the mercury in the thermometer from 50 dregrees to O; that is, will produce a cold 32° below that of freezing water. If 2 parts of snow or pounded ice be mixed with 1 part of common salt, the mercury will sink to 5° below O, and the temperature is then expressed thus-5°, the algebraic sign-or minus denoting that the degree of cold is less than that at the beginning of the scale. If three parts of muriat of lime (or fixed sal-ammoniac) be mixed with 2 parts of snow, the fluid in the thermometer will sink from 32° above Oto-50° below it; but the most intense cold hitherto known has been produced by mixing 10 parts of sulphuric acid (or oil of vitriol) diluted, with 8 parts of snow, by which the fluid will sink to-91° or 123 degrees below freezing water. To measure the intenseness of cold of this sort, spirits of wine or some essential oil must be employed instead of mercury, which itself freezes and becomes solid when the temperature falls to thirty-nine degrees below O. Nor can mercury measure heat greater than 660 degrees, for there it boils and is converted into vapour. thermometer in general use in France and other parts of the continent, was invented by the celebrated naturalist Réaumur: in it the point of freezing water is the beginning of the scale or zero, and the space up to boiling water is divided into eighty equal parts. Later philosophers abroad have divided that space into 100 equal parts, whence the thermometer is said to be centigrade. As considerable embarrassment is often occasioned by these different modes of measuring the temperature of the air and other bodies, the following rule for converting Reaumur's degrees into Fahrenheit's will be useful. Multiply the number of degrees of heat shown by Reaumur by 9, divide the product by 4, and to the quotient add 32, and the sum will be the degrees on Fahrenheit; and by reversing the operation degrees on the latter will be turned into those on the former. On our common thermometers different points of heat are marked, as at thirty-two degrees freezing; at fifty-five degrees temperate, at seventy-six degrees summer heat; at ninety-eight degrees blood-heat; at 112 degrees fever-heat; at 176 degrees alcohol (spirit of wine) boils; and at 212 degrees water boils.

The

How far our atmosphere may extend beyond the surface of the earth is unknown; it is observed, however, that the sun's rays passing through it at an elevation of forty-five degrees undergo no sensible refraction; so that the air at that point must be at least inconceivably rare. Nevertheless, meteors (balls of fire and shooting stars as they are called,) have been observed at an elevation of seventy or eighty miles, and most probably have been within the bounds of our atmosphere. Clouds seldom rise above 24 miles above the level of the sea; for there the atmosphere is equally rare with themselves.

If a lighted candle be placed under the receiver of the air-pump, it will burn as long as the air is sufficient for its support, and when it goes out, the smoke will mount up and remain in the top of the receiver like a cloud. If now the air be extracted by the pump the smoke will gradually sink down to the bottom of the receiver, leaving the top perfectly clear. This experiment exhibits the cause of the rise and fall of clouds and vapours in the atmosphere, and that smoke is not devoid of weight. A piece of wood floats upon water; yet no one ever concluded wood to possess no real weight or gravity.

Air differs from water and other liquid substances in this particular, that to whatever quantity it may be reduced and extracted, the remainder in the vessel will always completely fill it. This is caused by the very great elasticity or spring of its particles, which makes them depart from each other in all directions, in proportion as pressure is removed. If three quarts of air be pumped out of a vessel holding a gallon, the remaining quart will expand over the whole space of the vessel: if on the contrary the vessel be filled with water, and three quarts drawn off, the remaining quart will just occupy the same space as when the vessel was full. In this manner air may be rarified or made thin: in a similar way, on account of its elasticity, it may be condensed or made thick, and be compressed into less space than it usually occupies. The machine by which this is done is called a condenser: it consists of a brass barrel containing a piston, with a valve opening downwards; so that when it is drawn up the air above passes through it, and when it is pushed down the valve shuts, and the air below is forced through a valve in the bottom of the barrel into a tube communicating with the receiver placed on the frame. Thus at every stroke of the piston more air is driven into the receiver, made of very thick strong glass, and kept down upon the plate by a strong wooden frame and screws. It is by the strong spring of air, when greatly condensed, that the air-gun works. A hallow ball, into which a great quantity of air has been forced, is so connected with the lock, that, when the hammer moves, a portion of the air rushes into the barrel and expels the bullet with very considerable force. The air in the ball may be sufficient for several successive discharges; but as the air becomes at each rarer and rarer, this force, it is plain, will proportionably decrease.

Winds. Water in motion in a current is a river, so air in motion becomes wind. If the air were uniformly of the same density, at the same height, the lighter parts always resting above the heavier, the lateral pressure being equal in all horizontal directions, the air would then always be at rest. On the other hand, if any portion of the air be heavier than the rest, it will descend, or if lighter it will ascend, until the balance be restored; whatever cause therefore alters the density of the air, must of course cause a motion in it, or wind. The density of air is changed by compression and heat; its elasticity or spring is increased by moisture; and electricity may have some effect of the same kind: but the principal cause of wind is certainly heat, by which the elasticity or repulsive power of the particles of air being increased, they retire farther and farther from one another. By this process the number of particles in a given space being diminished, the total gravity is also lessened; the thin or rare heated air is therefore obliged to give way before the pressure of the adjacent denser cold air, and a motion or current, that is wind, is produced, more or less powerful in propor

tion to the different densities of the different kinds of air. The different winds are denominated from the points of the compass from which they blow; thus a north wind blows south, and an east wind blows west. They are also divided into variable and trade winds; the latter being subdivided into permanent and periodical. Permanent trade-winds blow constantly from the same point, or at least they are subject to very little variation. Periodical or shifting trade-winds, otherwise called Monsoons (from an Asiatic term signifying seasons), blow for a certain time in one direction, and then change to blow from the opposite point for a certain time. Variable winds, blow occasionally from every quarter of the heavens, without any regularity or uniformity of duration or strength.

The air over that part of the earth where the sun is vertical, is by his rays greatly heated and rarified; it therefore rises, and the adjacent cool air rushes in to supply its place. In consequence however of the earth's daily rotation on her axis from west to east, the sun apparently moves in the contrary direction; the rarified air will therefore move from east to west, and an easterly wind will be excited in the regions about the equator. Since the same places are brought round under the sun once every twenty-four hours, the air is not wholly cooled during the night, and thus a constant stream of air or wind from east to west is maintained. From the equator to about thirty degrees of latitude on each side, the wind blows from the eastward, but drawing more from north-east and south-east, in proportion as the place is situated more or less towards the north or the south of that line. This is the general state of the trade-winds, so called because they are of great service to commerce, by forwarding vessels on their voyages. Within the limits of the trade-winds, their direction, from local causes, is found to deviate considerably from the proper east; sometimes indeed even to the opposite quarter. Thus upon the coast of Guinea in Africa, although within the tract of the trade-winds, the wind blows from S. W. to N. W. The cause of this is, that the soil of that coast being in general sandy, it becomes extremely hot by the sun's rays; which greatly rarifies the air over the land, and then the cooler sea-air rushes in from the south and west to supply its place. At that part of the sea where this change of direction is perceived, there is generally a calm; and the air moving away east and west, what remains is unable to support the vapourse there collected, which accordingly fall down in frequent torrents of rain, so much so, that those parts are commonly called the Rains. By the Spaniards and Portuguese, they are called the Ladies' sea, because gales of wind are there very rare: to the mariner, however, such ealms are much more distressing than a gale, because they retard his voyage; and occasion a great waste of time and provisions to no purpose. The periodical trade-winds or monsoons blow regularly, during six months, from one quarter, and during the remaining six months from the opposite quarter. These winds prevail chiefly in the ocean on the south of Asia; for those countries lying on the north side of the equator, are greatly heated in summer, which occasions the wind to set in northward from the sea; but in the winter months, the air over the sea is more heated, and the wind again sets southward from the land. The position of the coasts gives particular directions to these winds; but the Indian monsoons usually blow from the S. W. during our summer, and from the N. E. during our winter hence ships going to, or coming from