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heat, when made of the same fluid. One may rise an inch, while another in the same place rises but half an inch.

The variation of two thermometers is directly as the capacities of their bulbs, and inversely as the bases of their stems. Suppose the diameter of the stems equal, but the capacities of the bulbs different: equal degrees of heat will swell and enlarge the bulks of the fluid, in proportion to their bulks: so that if the capacity of the one were double or triple that of the other, it would be enlarged twice or thrice as much; and if by swelling they rise in tubes of equal diameters, the largest must rise twice or thrice as high as the least. This is indeed upon the supposition, that the fluid is equally heated through in both. But on sudden changes of heat and cold, the case will be different, for the lesser ball will be sooner heated through, and be more expanded, in proportion to its bulk; unless the heat continues for some time, until it has penetrated the larger bulb also. So that on this supposition the rise of the fluid will be directly as the capacity of the bulb. Suppose, again, that the capacities of the bulbs were equal, but the diameters of the stems different. When the fluid is equally heated in both, equal quantities of it must rise in the stems, and therefore the heights to which they rise, must be greater in proportion as the diameters are less; that is, the heights will be inversely as the bases of the stems. Therefore when neither the capacities of the bulbs, nor the diameters of the stems are equal, the heights of the fluid in the thermometers will be directly as the capacities of the bulbs, and inversely as the bases of the stems.

Yet the variations of different thermometers may be compared together by some common scale, such as that

of Fahrenheit, in which the freezing point is at 32, and the heat of boiling water at 212 degrees. If any other thermometer, however different in the diameter of the stem, or capacity of the bulb, have its scale divided in the same manner, the degrees of one will be proportional to the degrees of the other, and indicate the same degree of heat, at any particular time, however different the lengths of the scales may be.

The thermometer is thus made. The tube must be well heated to expel the air out of it, and when the end of it is immersed in the fluid, the pressure of the air on the surface of the fluid will force it into the bulb of the thermometer, until that be filled, and part of the stem, as the tube begins to cool. The tube thus filled is immersed in snow, just beginning to melt, or water beginning to freeze, and the point, at which the fluid stands in this situation, is marked 32, according to Fahrenheit's scale. It is then immersed in boiling water, if it be filled with mercury, and the point at which it then stands is marked 212 degrees, the intermediate space being divided into 180 parts called degrees. At 96, the heat of the blood is noted; and the part of the tube below the freezing point is divided in the same manner. If spirit of wine be used, it will not bear so great a degree of heat as boiling water, before it would boil and burst the tube; the thermometer is therefore immersed in water heated to such a degree, as to suffer melted wax poured on it to begin to coagulate, and the point at which the spirit then stands is to be marked 142 degrees. The fluid is then to be raised to the greatest height, that it can bear, and the tube sealed at that point, to exclude the pressure of the external air.

As spirit of wine boils and bursts the tube with a small degree of heat, such a thermometer cannot indi

cate the heat of boiling oils, or melted metals. But as linseed oil acquires four times as much heat before it boils, as spirit of wine, its use is more extensive, but it soon sullies the tube, and reaches no farther than the heat necessary for melting lead or tin. It therefore gave way to the mercurial thermometer, which sustains a much greater degree of heat and cold. But this also has its limits, as the mercury has been lately found fixed by a high degree of artificial cold, and rendered malleable, or capable of being stretched under the ham

mer.

The bulb of the thermometer should be made cylindrical rather than spherical, that the heat may sooner penetrate the fluid, and thereby the sooner show the variations of heat and cold. For the mercury has been found to sink in the tube upon a sudden application of heat, before it rose; which was owing to the expansion of the tube, before the heat reached the center of the mercury.

9. Air is necessary for the existence of flame and fire; as a candle and live coals are suddenly extinguished by withdrawing the air from around them: it is necessary to promote putrefaction and fermentation; as animal substances, such as flesh and eggs, are preserved from putrefaction for a considerable time in vacuo, and by covering them with lard or butter, or any other substance that would exclude the air from them: and I need hardly add that it is absolutely necessary for the preservation of animal life, when the foramen ovale is closed. Nature has provided a double mode of circulation of the blood, for those animals, that are to live in air and water alternately, one through the lungs, while they are in the air, and the other, through the foramen ovale, when they are in the water.

Having thus considered the nature and general properties of fluids, both incompressible and elastic, we now proceed to that part of hydrostatics, called hydraulics, which explains their properties and operations, when they are in motion.

HYDRAULICS.

HYDRAULICS is that branch of natural philosophy, which considers and explains the laws and operations of fluids in motion, whether they be compressible or incompressible; the construction of machines for putting them in motion, or for being moved by them; the nature of winds and sounds, of springs and tides.

The motion of a fluid arises, generally, either from its gravity, pressure, or elasticity; or from the action. of heat, the force of machines, or the principle of gra

vitation.

The most natural order of treating this subject, is, first, to consider the motion of such fluids as are in-compressible, and secondly, the motion of such as are elastic.

MOTION OF INCOMPRESSIBLE FLUIDS.

The first cause of motion in fluids, which we shall consider, arises from their gravity, by which the particles that stand highest, press upon those below them; whereby the whole surface becomes level, or rather a part of a sphere, whose radius is the same with that of the earth. Thus water in the reservoir will rise in a small duct or canal that communicates with it, to the same height, if the canal be close all round, and continued up to the altitude of the reservoir; otherwise it will run out in a continued stream, until the water in the reservoir sink down to the same level. The solution of this

phenomenon depends upon this principle of hydrostatics,-that the pressure of fluids is in proportion to their perpendicular altitude, and not according to their quantity. Hence, water is often brought in canals under ground, from a reservoir in an elevated situation, to supply gardens, meadows, and cities, and distributed through various parts that are lower than the fountain. Hence also is the descent of waters in rivers and streams, from springs that are situated above them; and the breaking out of streams from the bottoms or sides of mountains, from cisterns in them, which are continually supplied by rains and vapours distilling through the earth. As a great quantity of the vapour raised from the surface of water returns to the earth in a silent and imperceptible manner, it was supposed to contribute but very little to the supply of rivers, until Dr. Halley instituted an experiment for estimating the quantity raised in the Mediterranean sea, supposing it to be 40 degrees long and 4 broad, on an average, in the course of a summer day. After salting some common water, and heating it moderately, until he brought it to the same degree of heat and specific gravity with the sea in a summer day, he suspended it to the end of a nice balance, and observed the quantity evaporated in a given time; from whence he determined the quantity evaporated from the surface of the whole Mediterranean sea, in a single day, to be, in round numbers, 5280 millions of tons. He next endeavoured to estimate the quantity of water furnished per day by all the large rivers, that empty themselves into this sea, supposing that each of them brought ten times as much as the river Thames; and found it to be only 1827 millions of tons; which is but about one third part of the quantity evaporated. The deficiency therefore must be made up by the rains

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