SECT. I.

OF FLUIDITY.

A FLUID body, in Sir Ifaac Newton's definition, is a body yielding to any force impreffed, and which has its parts very easily moved one among another. This is the definition of a perfect fluid: if the fluid require fome fenfible force to move its parts, it is an imperfect fluid; and the imperfection is in proportion to that force: fuch perhaps are all the fluids with which we are acquainted.

Fluids are either elaftic, fuch as air; or non-elaftic, as water, mercury, &c. The latter are incompreffible, and occupy the fame fpace under all preffures or forces; but the former dilate and expand themselves continually, by taking off the external preffure from them. The properties of the former fluids conftitute the doctrine of Pneumatics, before treated of; the latter contain the principles of Hydrostatics.

Fluidity differs from liquidity, or humidity; the latter implying wetting or adhering. Thus, air, ether, mercury, and other melted metals, and even fmoke and flame, are fluid bodies, though not liquid ones; while water, beer, milk, &c. are both fluids and liquids.

The modern opinion concerning the original and conftituent parts of fluids, is, that they are fmall, fmooth, hard, globular particles; confequently, each particle must be a folid globular body; and confidered fingly, is no fluid; but be comes a fluid, by being joined with other particles of the fame or a fimilar kind.

That the particles of fluid bodies are very fmall, is evident, from their texture having never been discovered by the finest microscope: that they are fmooth, appears from that freedom wherewith they glide over one another, when fet in mosion that they are hard and impenetrable, is plain from their being

VOL. II.

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being incapable of compreffion: and that they are spherical, is obvious, from their being fo eafily put in motion; and from the interstices or vacancies, which are hereafter proved to fubfift between them; which could not be the case, unless they were spherical, and touched each other only in fome fingle points of their furfaces. For, upon mixing falt with water, a certain quantity of the falt will be diffolved, without increafing the dimenfions of the water; which demonstrates the vacuities between the particles of the water. When a fluid becomes more buoyant, it is a proof that its specific gravity is increased, and consequently, many of its vacuities filled up; and even then it may receive a certain quantity of other diffoluble bodies, the particles whereof are adapted to the remaining vacancies, without adding any thing to its bulk, though the abfolute weight of the whole fluid be thereby increased. This is demonftrated by taking the weight of a phial of rain water with a nice balance: when the water is poured out, and some salt added to it, and the phial again filled with the water, it will be found to weigh more than when before the falt was put in, from the vacuities of the fresh water being filled with faline particles.

It has also been found by experiment, that the particles whereof fluids are compofed, confift of fpheres of different diameters, whofe interftices may be fucceffively filled with proper ingredients; and where these interftices are smaller, the gravity of the fluid will be greater, and vice versa.

For example: if a barrel be filled with any large spherical bodies, as bullets, many fmall fhot may afterwards be placed in the interstices of these bullets; the vacuities of the shot may then be filled with fea fand; the interftices of which may again be filled with water, which will also admit of a certain quantity of falt in the vacuities; and thus the weight of the barrel may be greatly increased, without increafing the space occupied by these materials. This reasoning alfo holds good in fluid bodies, as well as in those which are folid; for river water will diffolve a certain quantity of

falt;

falt; after which it will diffolve a certain quantity of fugar; and after that a certain quantity of alum; and then perhaps will receive other diffoluble bodies, without increafing the dimenfions of the whole.

If fluids were not compounded of fuch primary particles, but made up of one homogeneous fubftance, equally dense, without confiftence, there would be no difference in their specific gravities, and all fluids would be of the fame weight, which is not the case.

That a fluid has vacuities, is evident from the following confideration, viz. if all space were absolutely full of matter, that matter must be either fluid or fixed. If it were fixed, no motion could poffibly be therein, as is evident from reason and experience; it must therefore be fluid. But a fluid without vacuities would be denfer, and confequently heavier, than all other fluids; and if denfer, all bodies will emerge and swim at the top, by hydroftatical laws, and there would be no fuch thing as gravity. But as gravity exifts, all space therefore cannot be filled, even with a fluid.

By the experiments of Borcelli, it has been demonftrated, that the constituent parts of all fluids, are not fluids them. felves, but confiftent bodies; and that the elements of all bodies are perfectly firm and hard. The incompreffibility of water, proved by the Florentine experiment, is a fufficient evidence that each primary particle of this fluid is a perfect impenetrable folid.

This famous experiment was firft attempted by the ingenious Lord Verulam, who enclofed a quantity of water in a piece of lead, and found, that the water would fooner make its way through the pores of the lead, than be reduced to lefs compafs, by any force that could be applied. This experiment was afterwards made at Florence, with a globe of filver; which being filled with water, and well clofed, was gently preffed, when a small quantity of water issued through the pores of the filver in the form of dew.

The fame experiment was afterwards made by Sir Ifaac Newton, and others, with globes, made of gold and other metals; all which experiments were attended with the fame phenomenon, and have tended to establish the above theory.

As a great many of the phenomena in hydroftatics depend upon gravity, it may be neceffary to mention the laws of gravity, concerning bodies immersed in fluids.

Gravity, in hydroftatics, as well as in the other arts, divided into abfolute and fpecific gravity.

Abfolute gravity is that force with which the body tends downwards; and is always in proportion to the quantity of matter in the body.

Specific gravity is the relative, comparative, or apparent weight of any body compared with that of another body, of the fame bulk or magnitude; and therefore fignifies, that gravity or weight which is peculiar to each kind of body.

A body is faid to be specifically heavier than another body, when it contains a greater weight than the other, under the fame bulk or dimenfions; and thus reciprocally that body, which contains a lefs weight than another under the fame bulk, is faid to be fpecifically lighter than the other body. Thus, if there be two equal spheres, fuppofe one foot in diameter each; the one of lead, and the other of wood: as the leaden one is found to be heavier than the wooden one of the fame fize, it is faid to be fpecifically heavier, that is, heavier in fpecies, or in kind; and the wooden one is specifically lighter.

Specific gravity is by fome called relative gravity, to dif tinguish it from abfolute gravity.

The Laws of the Specific Gravities of Bodies.

1. Two bodies of equal buik have their specific gravities to each other, as their abfolute weights or denfities,

2. Two bodies of the fame fpecific gravity or denfity, have their abfolute weights in proportion to their magnitudes or

bulks.

3. The specific gravities in bodies of the fame abfolute weight are reciprocally as their bulks.

4. The specific gravities of all bodies are in a ratio compounded of the direct ratio of their weights, and the reci procal ratio of their magnitudes.

5. The abfolute gravities, or weights of bodies, are in the compound ratio of their specific gravities, and magnitudes or bulks.

6. The magnitudes of bodies are directly as their weights, and reciprocally as their specific gravities.

7. When a body is immersed in a fluid that is specifically lighter than the body, the body lofes as much of its weight as is equal to the weight of a quantity of the fluid of the fame bulk or magnitude.

Therefore, the specific gravities of two bodies are as the abfolute gravities, under the fame bulk. The specific gravity of the fluid will be to that of the body immersed in it, as the part of the weight loft by the folid is to the whole weight. And the specific gravities of two fluids are as the weights loft by the fame folid immersed in them.

8. The specific gravity of any fluid or folid body is found as follows:-Sufpend a globe of lead by a fine thread from one arm of a balance, and to the other arm fasten an equal weight. Immerse the globe of lead into the fluid, and observe what weight it will require then to balance it, and confequently what weight it has loft, which is proportional to the specific gravity of the fluid, compared with the whole weight of the folid. Thus the proportion of the specific gravity of one fluid to another, is determined, by immerfing the globe fucceffively in all the fluids, and obferving the. weight the globe has loft each time, which will be the proportions of the fpecific gravities of the fluids.

This operation alfo gives the specific gravity of the folid immerfed, whether it be a globe, or of any other shape, if the gravity of the fluid be known. For the fpecific gravity of the fluid is to that of the folid, as the weight loft is to the whole weight. 9. Th

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