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into the ball, and retained there by a valve. The piston is then drawn down, and creates a vacuum in the tube, until it arrives at a certain part where there is a hole, through which the air rushes and fills the tube, which is again forced into the ball: this action is repeated until the ball is sufficiently charged. By the condensation of the air, a very considerable degree of heat is generated. Tinder and fungus may be lighted, in a proper condenser, by the sudden stroke of the air.

The BAROMETER, a well-known instrument for exhibiting the approaching change of the weather, which has been already referred to, requires a slight explanation. In its most simple form, it is a tube of glass, of about 30 inches long, hermetically sealed at one end, filled with quicksilver, and inverted into a vessel of the same metal, when the quicksilver in the tube will descend until it arrives at from between 28 to 31 inches above that in the vessel, according to the state of the atmosphere. If the quicksilver in the tube falls or becomes low, it indicates rain; if it rises, it indicates fine weather. The principle, which has been already alluded to, is, that a column of air, of the whole height of the atmosphere, will balance a column of quicksilver of from 28 to 31 inches, according to the state of the air. When the atmosphere is loaded with aqueous vapour, it becomes lighter, aqueous vapour being considerably lighter than atmospheric air; consequently it will not balance so large a quantity of quicksilver, and the quicksilver of course sinks. When the aqueous vapour is condensed into rain, and yields its place to the common atmospheric air, the quicksilver again rises. Barometers are made in different ways, but the principle is invariably the same. Barometers are used to ascertain the height of mountains or other elevated situations, the mercury sinking one-tenth of an inch at about every hundred feet of

ascent.

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WATER.

Water a compound body-Different states of water-Water universally diffused-Salt-water-Snow-water-Rain-water-River-water-Springwater-Expansive power of water converted into steam-Theory of boiling, &c. &c.

WATER is a clear, colourless liquid, and if pure, is without either taste or smell. It used to be imagined by the ancients that water was an element or simple body; but it is now well known to be a compound of two gases, oxygen and hydrogen, in the proportion of about 85 parts of the former to 15 of the latter by weight.*

Water exists in four distinct states: in Ice, Water, Vapour, and in combination with other bodies. Ice is the simplest state of water, it being water deprived of the greatest portion of its caloric. When water receives an excess of caloric, it becomes vapour or steam.

Water is most universally diffused, for there is scarcely any body in nature that does not contain it: wherever there is matter there appears to be water. Hartshorn kept fifty years, so as to be as dry and hard as any metal, and even capable of producing sparks, like steel when struck with a flint, will, if distilled, produce one-eighth of its weight of water; and bones that have been dried for twenty-five years have produced one-half of their weight of water. The hardest stones ground and distilled discover a portion of water; and it has been said that even fire itself is not without it.

The grand reservoir of water is the Ocean: this covers nearly three-fifths of the earth's surface, and extends to an unfathomable depth. The most striking property of sea-water is its saltness, the cause of which philosophers have endeavoured to account for in various ways. Dr. Halley supposed that the sea was originally fresh, and that its saltness proceeded from the salts of the earth conveyed into it by rivers. Others have supposed that there are

* Water contains air holding a larger quantity of oxygen than atmospheric air; fishes breathe this air, which if deprived of they die."

vast rocks of salt at the bottom of the sea, whence it derives its saltness; and this is rendered more plausible from the numerous and extensive salt-mines known to exist in different parts of the earth. Others, again, have supposed that the sea was created salt to keep it from putrefaction; but it appears to be kept from putrefaction by the agitation of the winds and tides, for stagnant saltwater will in time become corrupt. Although the sea is universally salt, yet it is salter within the tropics than at the arctic regions; this is accounted for by the quantity of snow, and the large rivers, that fall into the northern seas, to which may be added, that evaporation is infinitely less in those seas than between the tropics, consequently a proportionately less quantity of fresh water is carried off.

Of the various kinds of water, Snow-water, when collected clean, is the purest. Rain-water may be considered the purest distilled water, but in its passage through the air, it becomes impregnated with putrescent matter, which, while it renders it superior to any other for fertilizing the earth, renders it inferior to spring and river water for various purposes. The manner in which rain-water is generally obtained, viz., from gutters placed under the roofs of houses, adds to its impurity, and renders it in a short time quite putrid.

River-water is next in purity to rain-water, particularly where it runs over a clean, stony bottom, and is superior to it for domestic purposes. River-waters putrefy sooner than spring-waters, but they soon throw off their heterogeneous matter, and become sweet, and purer than before, and continue so for a long time. On this account it is that the Thames water is generally preferred for long voyages.

Spring-water is impregnated with a portion of neutral salts, which makes it hard; it is also occasionally impregnated with iron. It is, however, preferred, as a beverage, for its coolness and pleasantness, though it is less wholesome than good river-water. Springs are formed by the intervention of clay and sand strata, the former holding water, and the latter allowing it a free passage; so that no water is found, in digging wells, until clay is penetrated. Springs of fresh water are found to exist even

under the sea. It is said that thirty have been discovered under the sea on the south of the Persian gulf. St. Winifred's Well, in Flintshire, is said to throw up 120 tons of water in a minute, and to turn eleven mills in the space of about a mile and a half. There are hot springs in various parts of the world, of which the Great Geyser, in Iceland, is the most remarkable. The chief hot-springs in England are those of Bath and Bristol: the hot baths of Bath are as high as 115° of Fahrenheit.

The specific gravity of water is constantly changing with the temperature of the atmosphere. At a mean rate, it is 820 times heavier than air; so that a cubic foot of water will weigh 1000 ounces avoirdupois very nearly.

The

Water is of a very volatile nature, and soon raised to vapour. If exposed to the action of the fire in an open vessel, it would entirely go off in vapour, and consequently occupy a space infinitely greater than it did before.* expansive force which water acquires when converted into steam is greater than that of gunpowder:† thus a pound of water converted into steam may be made to displace or raise a greater weight than a pound of gunpowder would. It is to this principle that the mighty power of the steamengine is indebted; and volcanic eruptions and earthquakes are supposed to be often caused by water meeting with subterraneous fires, and being suddenly converted into steam. It is through the expansive power of steam that that well-known motion called boiling is caused. The theory of heating and boiling is as follows:-When a vessel of cold water, for instance, is placed over the fire, that layer of water nearest the fire will become heated first, and will consequently, through its expansion, acquire a specific gravity different from that of the mass; the heated portion will ascend, and its place be occupied by

* According to Count Rumford, steam at 212o is 3000 times rarer than water; but with a great heat, it may be made to occupy more than 10,000 times the space of water.

+ This is strikingly evident in Perkins's steam-gun, which will discharge seventy balls in four seconds.

Some steam-engines are sufficiently powerful to raise a weight equal to forty millions of pounds one foot, and that with a single bushel of coals.

another portion, and thus the whole mass will be brought successively into contact with the heated bottom of the vessel, and continue to have its temperature increased until the caloric or heat generates vapour so rapidly and violently that it rushes up and causes the boiling. So that, in this operation, there is a constant circulation of the water about the vessel, that which was nearest the bottom, being converted into vapour, ascends, giving way to that which was immediately above it. The degree of heat to produce boiling depends on the weight or pressure of the atmosphere. When the barometer is at 29 inches, water boils at about 210°. When the barometer is at 31 inches, the boiling point is rather more than 215°. It is not through the conductive power of water that its temperature so soon becomes raised; water being so bad a conductor of heat, that a portion of water, in a glass tube containing ice, may be made to boil without even melting the ice.

Water is the most penetrative of all bodies, and the most difficult to confine; passing through leather, and even bladders, which will confine air. It used to be supposed that water is incompressible: from recent experiments, it is found that water may be compressed, but not to any considerable extent.

HYDROSTATICS AND HYDRAULICS.

Hydrostatics and Hydraulics defined-Pressure of Fluids and peculiar properties-Specific gravity of Bodies-Motion of water in pipes-Sucking Pump-Forcing Pump-Siphon-Jets, &c.

HYDROSTATICs is that science which treats of the nature, gravity, and equilibrium of fluids, and of the weighing of bodies therein. Hydraulics treats of the force and velocity of fluids in motion.

Fluids are those bodies which possess a perfect freedom of motion, and whose parts yield to any impression. They are divided into elastic and non-elastic. Air, vapour, and gases, are elastic, as their volume may be diminished by pressure. Mercury, water, &c. are non-elastic or incompressible, except to an inconsiderable amount.*

* Perkins is said to have compressed water one-twelfth, with 2000 atmospheres.

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