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is adopted as a standard, we may denominate its weight by any number we please ; but then the weight of all bodies tried by this standard must be signified by proportional numbers. If we call the weight of water, for example, l, then that of gold would be 19; or, if we call the weight of water 1000, that of gold would be 19,000. In short, the specific gravity indicates how much more or less a body weighs than an equal bulk of

water.

SPRINGS, FOUNTAINS, &C.

The water belonging to our globe exists in various states. It is the same water which successively forms seas, rivers, springs, clouds, rain, and sometimes hail, snow, and ice. When the first rays of the sun warm the surface of the earth, the heat, by separating the particles of water, transforms them into vapour, which, being lighter than the air, ascends into the atmosphere. The atmosphere diminishing in density as it is more distant from the earth, the vapour which the sun causes to exhale, not only from seas, rivers, and lakes, but likewise from the moisture on the land, rises till it reaches a region of air of its own specific gravity, and there it remains stationary. By the frequent accession of fresh vapour, it gradually accumulates, so as to form those large bodies of vapour which we call clouds; and these at length, becoming too heavy for the air to support, fall to the earth in the form of rain. If the watery particles retained the state of vapour, they would descend only till they reached a stratum of air of their own specific gravity; but during their fall, several of the watery particles come within the sphere of each other's attraction, and unite in the form of a drop of water. The vapour, thus transformed into a shower, is heavier than any part of the atmosphere, and consequently descends to the earth. Observe, that if the waters were never drawn out of the earth,

vegetation would be destroyed by the excess of moisture ; if, on the other hand, the plants were not nourished and refreshed by occasional showers, the drought would be equally fatal to them. Were the clouds constantly in a state of vapour, they could never fall to the ground; or were the power of attraction more than sufficient to convert the vapour into drops, it would transform the cloud into a mass of water, which, instead of nourishing, would destroy the produce of the earth. We cannot consider any part of Nature attentively without being struck with admiration at the wisdom it displays: we cannot contemplate these wonders without feeling our hearts glow with admiration and gratitude towards their bounteous Author.

Water, then, ascends in the form of vapour, and descends in that of rain, snow, or hail, all of which ultimately become water. Some of this falls into the various bodies of water on the surface of the globe, the remainder

upon the land. Of the latter, part re-ascends in the form of vapour, part is absorbed by the roots of vegetables, and part descends into the bowels of the earth, where it forms springs. The only difference between rain and spring water consists in the foreign particles which the latter meets with and dissolves in its passage through the various soils it traverses. Spring water being more pleasant to the taste, and more transparent, is commonly supposed to be more pure than rain water. Excepting distilled water, however, rain water is really the most pure we can obtain ; it is this which renders it insipid, whilst the various salts and different ingredients dissolved in spring water, give it a species of flavour, without in any degree affecting its transparency; and the filtration it undergoes through gravel and sand in the bowels of the earth cleanses it from all foreign matter which it has not the power of dissolving.

When rain falls on the surface of the earth, it continues making its way downwards through the pores and crevices in the ground. Several drops meet in their subterraneous passage, unite, and form a little

rivulet : this, in its progress, meets with other rivulets of a similar description, and they pursue their course together in the interior of the earth, till they are stopped by some substance which they cannot penetrate ; for though we have said that water under strong compression penetrates the pores of gold, when acted upon by no other force than gravity, it cannot make its way even through a stratum of clay. This species of earth, though not remarkably dense, being of great tenacity, will vot admit the passage of water. When, therefore, it encounters any substance of this nature, its progress is stopped, and the pressure of the accumulating waters forms a bed, or reservoir,

The next figure represents a section of the interior of a hill or mountain. A is a body of water such as has been described, which, when filled up as high as B (by the continual accession of waters it receives from the ducts or rivulets a, a, a, a,) finds a passage out of the

DESI WO og

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cavity; and, impelled by gravity, runs on, till it makes its way out of the ground at the side of the hill, and there forms a spring, c. The spring, during its passage from B to c, rises occasionally, upon the same principle that water rises in the spout of a tea-pot, but it cannot mount above the level of the reservoir, whence it issues ; it must therefore find a passage to some part of the surface of the earth that is lower or nearer the centre than the reservoir.—Water may thus be conveyed to every part of a town, and even to the upper stores of the houses, provided that it be originally brought from a height superior to any to which it is conveyed.

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Reservoirs of water are seldom formed near the summit of a hill, for in such elevated situations there can scarcely be a sufficient number of rills to supply one; and without reservoir there can be no spring. In such situations, therefore, it is necessary to dig deep wells, in order to meet with a spring; and then it can rise in the well only as high as the reservoir whence it flows.

When reservoirs of water are formed in very elevated situations, the springs which feed them descend from higher bills in the vicinity. There is a lake on the very summit of Mount Cenis which is supplied by the spring of the bigber Alps surrounding it.

A syphon is an instrument commonly used to draw off liquids from large casks or other vessels which cannot be easily moved. It consists simply of a bended tube. If its two legs are of equal length, and filled with liquid, if held perfectly level though turned downwards, the liquid will not flow out, but remain suspended in the tube ; for there is no pressure of the atmosphere above the liquid, while there is a pressure from below

upon

the

open ends of the tube ; and so long as this pressure is equal on both ends, the liquid cannot How out; but if the smallest inclination be given to the syphon, so as to destroy the equilibrium of the water, it will immediately flow from the lower leg. When syphons are used to draw off liquids, the legs are made of unequal lengths, in order to render the pressure of the liquid unequal; the shorter leg is immersed in the cask, and the liquid lows out through the longer. To accomplish this, it is however necessary to make the liquor rise in the shorter leg, and pass over the bended part of the tube, which is higher than the level of the liquor in the cask. There are two modes of doing this: one is, after immersing the shorter leg in the liquor to be drawn off, to suck out the air of the tube from the orifice of the longer leg; then the liquor in the cask, which is exposed to the pressure of the atmosphere, will be forced by it into the tube which is relieved from pressure. As

long as the tube continues full, no air can gain admittance ; the liquor will therefore flow on till the cask is emptied. The other mode is to fill the syphon with the liquor, then stopping the two ends with the fingers, immerse the shorter leg in the vessel, and the same effect will follow. In either case, the water in the highest part of the syphon must not be more than 32 feet above the reservoir ; for the pressure of the atmosphere will not support a greater height of water.

The phenomena of springs which flow occasionally, and occasionally cease, may often be explained by the principle of the syphon. The reservoir of water which supplies a spring may be considered as the vessel of liquor to be drawn off, and the duct the syphon, having its shorter leg opening in the reservoir, and its longer at the surface of the earth whence the spring flows; the water cannot be made to rise in the syphon by either of the artificial modes which we have mentioned, the spring will not begin to flow till the water in the reservoir has risen above the level of the highest part of the syphon: it will then commence flowing upon the principle of the equilibrium of fluids ; but it will continue upon the principle of the syphon; for, instead of ceasing as soon as the equilibrium is restored, it will continue flowing as long as the opening of the duct is in contact with the water in the reservoir. Springs which do not constantly flow are called intermitting, and are occasioned by the reservoir being imperfectly supplied.

but as

ON THE MECHANICAL PROPERTIES OF AIR.

We shall now examine the second class of fluids, distinguished by the name of aëriform, or elastie fluids, the principle of which is the air we breathe, which surrounds the earth, and is called the atmosphere. There is a great variety of elastic fluids, but they differ only in their chemical, not in their mechanical properties; and

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