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The specific gravity of a body, either fluid or solid, is ordinarily found by means of the hydrostatic balance; a most ingenious device for exactly ascertainingthe weight, either immersed in the water, or in the air. The construction of this instrument requires peculiar nicety, but it may be appended to any common balance ; as will be understood from the following description. Each scale should have a small hook fixed to the centre of its bottom, or lower side; so these small weights may be attached by means of horse-hair, or fine silk, thence to suspend a body in water without wetting the scale. First weigh the body in the usual manner in the scales, with great exactness; immerse it in water, and the equilibrium will be instantly destroyed. To restore it, put into the scale, from which the body immersed in the water is suspended, as much weight as will bring it even with the other scale, in which the opposing weight remains unaltered. The added weight will be equal to that of a quantity of water equalling the immersed body in bulk. Now, if the weight of the body in air be divided by what it weighed in the water, the quotient will show how much that body is heavier than its bulk of water. A guinea, new from the mint, will require 129 grains to be offered to its weight in air; but, on being immersed in water, will require 74 grains more to restore the equilibrium lost by the immersion. From this we see, that a quantity of water equal in bulk with the guinea weighs 74 grains, or 7.25, by which divide 129, (the weight in air,) and the quotient will be 17.793, shewing that the guinea is as 17.793, to one of water. But we sometimes have occasion to ascertain the precise weight of bodies that are lighter than water, say a piece of cork, and which, if unaided, would float on its surface. In such case, it is necessary to affix a weight (having previously found its exact poise) thereto ; when, by immersing both, and deducting the amount of the collateral weight, the residue will be left to account of the cork. If you would weigh quicksilver, it must be first balanced in a glass bucket, of which the weight is known, and which has been weighed also by immersion. When the bucket has been brought to equilibriumin the water, pour in the quicksilver, and the additional weight requisite to counterbalance it will show its exact weight. o: the following general rules for finding the specific gravity of bodies may prove useful and familiar to every under

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standing. 1, “When the body is heavier than water.” Weigh it both in water, and in the atmosphere, and the differ. ence between the results will show the quantity lost in, the former mode; then, as the weight lost in water is to the weight in air, so is the gravity of water to the gravity of the body. 2. “When the body, being specifically lighter, will not sink in water.” Render the body heavy enough to sink by means of some appendage, as a small piece of lead, &c.; weigh the body and the appendage, both separately and together, in the air, and in the water; find out how much each loses in the water, and subtract those losses from the whole weight of each in air. Then, as the last remainder is to the weight of the light body in air, so is the gravity of water to the gravity of the bo. dy. 3. “When a fluid is to be weighed.” Weigh the fluid in a cup, which is to be deemed an appendage, and treated according to the foregoing rule, observing, that as the whole weight is to the loss of weight, so is the gravity of the solid to the gravity of the fluid. - . We may ascertain the respective weights of two known ingredients in a given compound thus: take the differences of every pair of the three specific gravities; (viz. the specific gravities of the compound, and of each ingredient :) multiply each quantity by the difference of the other two; then, as the greatest product is to the whole weight of the compound, so is each of the other two products to each respective weight of the two ingredients. If a piece of glass, or of metal, be immersed by suspension in different fluids, it will lose in weight; that is, it will require an equipoise, according to the weight of the fluids respectively: observing, that in the lightest fluid, say alcohol, it will lose least weight. This is the principle on which the hydrometer acts, as will be subsequently shown. Vessels filled with water weigh more than when empty: to prove this, let a bottle be loaded so as to sink in a pail of water, deep enough for the water to cover its mouth, which should be previously closed by a plug, in such manner as might be easily pushed in ; append the bottle in equilibrio to the hydrostatic balance, and drive in the plug: the water will follow and destroy the equilibrium. Fluids press every away alike, though their general tendency is to gravitation. Thus, if a vessel be made weaker in the side than at the bottom, and be so laden or oppressed by the weight of water, as to burst the vessel, the weakest part, wherever situated, will become the outlet; but, so soon as liberated, the fluid will invariably descend, unless acted upon by a syphon, as shown in treating of hydraulics. The pressure upwards is, however, merely in conformity with circumstances attendant upon general pressure, and proves the tendency of fluids to find their own level. Thus, if you take a glass tube of moderate diameter, open at both ends, and stop one closely with your finger, when you immerse the other end in any fluid, it will enter but little within the vacancy; because the columns of air within the tube repress it. But when the finger is withdrawn, the water will ascend within the tube, to the level of the body in which it is immersed. As fluids press in all directions, it is evident their whole weight cannot be applied against one part or side; while on the other hand it is equally true, that, in some instances, the bottoms of vessels receive a pressure which does not appear to be their due. base is narrower than its brim, the bottom sustains only the weight of a column equal to its area, multiplied by its height; yet if the pan be of a bell-shape, having its base broader than its brim, the bottom will sustain a weight equal to its area,also multiplied by its height. Consequently, in a vessel of a conical form, the base would be opressed as much as if the sides were cyindrical. This is called the hydrostatic paradox; but will be easily reconciled by the consideration, that if a tube of glass be made with a curved bottom, so as to turn up in the form of the letter U, but with one leg or part much wider than the other, the water will rise equally in both. If to each a piston be fitted, their weights

being equal, and that one piston be first

put into the wider leg of the tube, it will cause the fluid to rise in the other in proportion to its weight; but on applying the lesser piston to the corresponding smaller tube, the two will be held in equilibrio. the pressure of water upwards, by means of two boards, whose sides are joined by leather, as in a pair of bellows; these may be of any form, or of any size. At the top of one of the boards cut a hole, and insert a tube of about four or five feet in length, so as to be perfectly tight: place on the board several weights, according to the size of the machine, and pour water into the tube. The upper board will bear up against the weights, provided they be not

Thus, in a pan whose

We have indeed further proof of

disproportionately heavy; and will admit the water between the top and botton to the extent admitted by the pliable sides. Some water ought to be poured in before the weights are set on. A circle of about twenty inches in diameter will thus list and support three weights, of 100th, each. Where either air or any other fluid is debarred from access between two pianks annexed in the water, the lower one being kept to the bottom forcibly, they will not separate, unless a force equal to the weight of the superincumbent fluid be applied; because the lateral and superior parts of the fluid are prevented from exerting their pressure, except in that direction which keeps the two planks together; but if the smallest opening be given, the pressure of the atmosphere will urge the fluid between them, and by confining it to act as a wedge, force the upper one to the surface. The comparative weights of fluids are ascertained by the Hypnow Eren, which see.

The comparative weight of fluids is given with the table of specific gravities,

(see GRAvity, specific); but it may be as

well to point out in this place, that a gallon of proof spirit weighs 7lb. 1202. avoirdupois.

If a vessel contain two immiscible fluids (such as water and mercury), and a solid of some intermediate gravity be immersed under the surface of the lighter fluid, and float on the heavier, the part of the solid immersed in the latter will be to the whole solid, as the difference between the specific gravities of the solid and of the lighter fluid is to the difference between the specific gravities of the two fluids.For a body immersed in a fluid will, when left to itself, sink, if its specific gravity be greater than that of the fluid ; if less, it will rise to the surface : if the gravities be equal, the body will remain in whatever part of the fluid it may be placed.— But in the case adverted to, the one fluid being heavier and the other lighter than the body immersed, it is necessary to combine their gravities by the mode above shown.

Balloons are properly hydrostatic machines, and derive their property of ascending from the earth into the upper part of our atmosphere entirely from the difference between the specific gravity of the air, or gas, with which they are filled, and the exterior, or atmospheric, air in which they float. The weight of the materials must be taken into consideration; for unless the specific gravity of the inte

rior be so much less than that of the exterior air, as to allow for the weight of the materials as a counterpoise, the balloon cannot be made to float even in a stationary manner; but when liberated will fall to the ground. The contents of the balloon being ascertained in cubic feet, it will be easy to ascertain what weight the balloon can lift, when filled with rarified air, according as that may have been rendered more light than the atmospheric air: if filled with gas, the interior will be at least seven times lighter than an equal quantity of atmospheric air. From this it will be seen, that to bear up a weight of 300th. the balloon must be large, and the specific gravity of its contents be adequate to overcome the resistance of that impediment. As the air of the upper part of our atmosphere becomes gradually more rare, and consequently lighter, according to its distance from the earth's surface, we may conclude that there is a point in its altitude, beyond which a balloon could not soar ; because its own weight, even if nothing were appended, would at such a point perfectly equipoise the difference between the confined gas and the surrounding atmosphere. And this is the move erfectly to be admitted, from the know!. we have acquired of the difficulty with which balloons are made to reach certain heights, and of their ascent being shown (by the slower fall of the mercury within the barometer) to be far slower in the upper regions when they approach that state of equipoise, Were it not for the opposition offered by the superior air, a balloon would rise instantaneously, from the moment of its liberation, in a most rapid manner, to that height where its equipose should be found. We have said thus much in explanation of the nature of the balloon, as appertaining to the laws of hydrostatics, referring the reader to the article Agnost Atrox, for whatever appertains to the practical experience we have had of that science, which at first seemed to promise the most important aid to various others, but in which it has completely failed : the whole of the principles on w!,ich aerostation depends have been long understood. We shall now speak of the diving-bell, which also depends on hydrostatic principles, though, like the balloon, it has a close connection with pneumatics. The upper part of a diving-bell is always made to contain a certain quantity of air, more or less compressed, in proportion to the depth to which the bell sinks. Thus, if

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bler, and occasion the air to be compress

ed into a smaller space. But the quantity of vital principle in the compressed air will be equal to that quantity of air in the open atmosphere which would fill the interior of the tumbler. If the inverted tumbler were first placed at the bottom of an empty vessel, and that water were afterwards poured into the latter, the ef. fect would be precisely the same. The air-contained within the upper part of a diving-bell not only debars the ingress of water, but, like the ratified air in the balloon, gives the machine such a buoyancy, that, unless made very substantial, and duly laden at the botton, or a broadest part, it would sink with difficulty, and be apt to turn on its side, so that the air would escape. Under the head of Drvaxe-hell, the reader will find an ample detail of the inventions hitherto extant in that branch of adventure. With regard to the depth to which floating bodies become immersed in fluids, we may consider the following general Principles, or propositions, to be sufficient for the purpose of our readers.Bodies, whose bases, or bottoms, are angular, like the keels of ships, will be *immersed deeper than those whose bases

are flat, such as barges: hence sharp-

built vessels necessarily (to use the technical term) “draw more water” than those of a more obtuse form : the reason of which is easily demonstrated, “is. As every body floating on a fluid will be imtnersed in proportion to its weight, and will displace a quantity of water equal thereto, it follows, that as a trianglé is equal to only half a parallelogram of equal base and altitude, a paralielogram (or flatbottomed vessel) will, under equal pressure, sink only half the depth of a triangular shaped bottom, of equal base and altitude For the same reason, vessels that have sharp stems make an easier passage though the water than such as are more “blufi,” or obtuse, “at the bows:” the

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more acute the triangle in that part, the less the resistance; for the triangle displaces only half the quantity of water that would be removed by a parallelogram of equal base and altitude; ergo, it would proceed twice as far, within a given time, as the latter, were not the friction, in some degree, increased. It must be obvious, that whether the vessel alone, or the circumstance of her being laden, cause her to weigh more than the quantity of water displaced by her whole bulk, up to the very gunwale, is not material; for in such case she cannot float, but must be depressed by the sum of specific gravity thus produced. This will appear in a very natural and simple manner, if we load a cup with small shot, &c. for, though partly empty, the cup will sink whenever the whole weight may exceed that of the water displaced. Both the cup and the shot are, however, specifically heavier than their bulk of water, and the former would sink if let in sideways; but then it would only displace a quantity of water corresponding with its own bulk, which would be trivial, when compared with that removed by its pressure as a floating body. On the other hand, we find that a slip may be laden with cotton,

which is far lighter than water, so as to

sink, at least to a level with the water, though not to precipitate to the bottom, unless forced by the adjunction, in whatever form or manner, of such other substances as are heavier than water, by which the levity of the cotton may not only be counterpoised, but exceeded. In India, where the principles of hydrostatics are absolutely unknown, the peasants make rafts of the straw, which they perceive to be lighter than water, and on them load the corn threshed from that straw, pereeiving it to be heavier than water. Thus they act upon the best principles, merely from observation' Perhaps, among the most curious circumstances that come within the verge of our subject, nothing can more fully exemplify what has been advanced, than the fact, well known, of some vessels sailing better upon than before the wind. We have no doubt that, if the forms of their bottoms were correctly ascertained, they would be found to present such a surface in the former position, when “keeled a little,” as created a more favourable position of the gravity of the vessel, though it must be at least equal, or, indeed, greater, if much pressed by the wind, than in the latter position. - - * Before we quit this subject, it is necesVOL. VI.

sary to inform the reader, that, except in cases relating purely to statics, few instances occur, in which the various matters appertaining to hydrostatics can be treated in a manner perfectly abstracted from pneumatics, or from hydrodynamics. Under the head of FLUIDs, and of HyIn Aulics, we have treated of the principles of fluids in motion, in such a way as may give a popular idea of those very intricate subjects; recommending to the student to read the whole contained under those articles with attention, and combining their several actions as derived from one great principle. HYDRO sulphuret, in chemistry, the combination of sulphuretted hydrogen with an alkaline or earthy base. The general properties of these substances are, that they are soluble in water, and are crystallizable ; the solution is colourless, while the action of the air is excluded; but when that is admitted, a yellow colour is soon acquired, owing to the oxygen of the atmosphere combining with the hydrogen of a portion of the sulphuretted hydrogen, while the sulphur combines with the remaining portion of it, forming a super-sulphuretted hydrogen, in union with the base. Mr. Murray observes, that “the knowledge which we have acquired of sulphuretted hydrogen, and of its combinations, has thrown light on the composition of the mineral sulphureous waters, and of the changes which they suffer. As sulphur is by itself insoluble in water, and as frequently no traces of an alkali, by which it might be rendered soluble, could be discovered in them, chemists found it difficult to conjecture by what means its solution was effected. The discovery of sulphuretted hydrogen, and of its solubility in water, solved the difficulty; and the mutual action exerted between it and the oxygen elucidate the changes these waters suffer from exposure to the air.” HY GROMETER, a machine or instrument to measure the degrees of dryness or moisture of the atmosphere. There are divers sorts of hygrometers; for whatever body either swells or shrinks by dryness or moisture is capable of being formed into an hygrometer. Such are woods of most kinds, particularly ash, deal, poplar, &c. Such also is catgut, the beard of a wild oat, &c. All bodies that are susceptible of imbibing water have a greater or less disposition to unite themselves with that fluid, by the effect of an attraction similar to chemical affinity. If we plunge into waF. c.

ter several of these bodies, such as wood, a sponge, paper, &c. they will appropriate to themselves a quantity of that liquid, which will vary with the bodies respectively; and as, in proportion as they tend towards the point of saturation, their affinity for the water continues to diminish, when those which have most powerfully attracted the water have arrived at the point, where their attractive force is found solely equal to that of the body which acted most feebly upon the same liquid, there will be established a species of equilibrium between all those bodies, in such manner, that at this term the imbibing will be stopped. If there be brought into contact two wetted or soaked bodies, whose affinities for water are not in equilibrio, that, whose affinity is the weakest, will yield of its fluid to the other, until the equilibrium is established; and it is in this disposition of a body to moisten another body that touches it, that what is called humidity properly consists. Of all bodies, the air is that of which we are most interested to know the different degrees of humidity, and it is also towards the means of procuring this knowledge that philosophers have principally directed their researches; hence the various kinds of instruments that have been contrived to measure the humidity of the air. A multitude of bodies are known, in which the humidity, in proportion as it augments or diminishes, occasions divers degrees of dilatation or of contraction, according as the body is inclined to one or other of these effects, by reason of its organization, of its texture, or of the disposition of the fibres, of which it is the assemblage. For example, water, by introducing itself within cords, makes the fibres twist, and become situated obliquely, produces between those fibres such a separation, as causes the cord to thicken or swell, and, by a necessary consequence, to shorten. The twisted threads, of which cloths are fabricated, may be considered as small cords, which experience, in like manner, a centraction by the action of humidity; whence it happens, that cloths, especially when wetted for the first time, contract in the two directions of their intersecting threads; paper, on the contrary, which is only an assemblage of filaments, very thin, very short, and disposed irregularly in all directions, lengthens in all the dimensions of its surface, in proportion as the water, by insinuating itself between the intervals of those same filaments, acts by placing them further asunder, proceeding from the middle towards'the edges. Different bodies have been em.

loyed successively in the construction of hygrometers, chosen from among those in which humidity produces the most sensible motions. Philosophers have sought also to measure the humidity of the air by the augmentation of weight undergone by certain substances, such, as a tuft of wool, or portions of salt, by absorbing the water contained in the air. But, besides that these methods were in themselves very imperfect, the bodies employed were subject to alterations, which would make them lose their hygrometic quality more or less promptly; they had, therefore, the double inconvenience of being inaccurate, and not being of long service. To deduce from hygrometry real advantages, it must be put in a state of rivalry with the thermometer, by presenting a series of exact observations, such as may be comparable in the different hygrometers. The celebrated Saussure, to whom we are indebted for a very estimable work on hygrometry, has attained the accomplishment of this object by a process of which we shall attempt to give some idea. The principal piece in this hygrometer is a hair, which Saussure first causes to undergo a preparation, the design of which is to divest it “of a kind of oiliness that is natural to it, and that secures it to a certain point from the action of humidity. This preparation is made at the same time upon a certain number of hairs forming a tuft, the thickness of which need not exceed that of a writing pen, and contained in a fine cloth serving them for a case. The hairs thus inveloped are immersed in a longnecked phial full of water, which holds in solution nearly a hundredth part of its weight of sulphate of soda, making this water boil nearly thirty minutes; the hairs are then passed through two ves. sels of pure water while they are boiling : afterwards they are drawn from their wrapper, and separated; then they are suspended to dry in the air; after which there only remains to make choice of those which are the cleanest, softest, most brilliant, and most transparent. It is known that humidity lengthens the hair, and that the process of drying shortens it. To render both these effects more perceptible, Saussure attached one of the two ends of the hair to a fixed point, and the other to the circumference of a moveable cylinder, that carries at one of its extremities a light index or hand. The hair is bound by a counter-weight of about three grains, suspended by a delicate silk, which is rolled in a contrary way about the same cylinder. In propor

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