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 HYDROMETER, 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 7 lb. 12 oz. avoirdupoise.

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 to the difference between the specific gravity of the air, or gås, 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 interior 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 300 lb. 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 more perfectly to be admitted, from the knowledge 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 equipoise 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 AEROSTATION, 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 which 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 we invert a small tumbler into a vessel nearly filled with water, and allow it to descend perpendicularly, so that no air may be allowed to escape, the water will rise a very little way within it. If the tumbler be but partially immersed, the water could at the utmost but rise to its own level; but if immersed so deep as to exceed its own interior, and that the bottom edge of the tumbler does not touch the bottom of the vessel, the water will, in consequence of its own greater, weight at a greater depth, rise rather, though scarce perceptibly, higher in the tumbler, and occasion the air to be compressed into a smaller space. But the quantity of vital principle in the compressed air will be equal to that quantity of air in the

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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 effect 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 rarified air in the balloon, gives the machine such a buoyancy, that, unless made very substantial, and duly laden at the bottom, or 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 DIVING-bell the reader will find an ample detail of the inventions hitherto extant in that branch of adventure.

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With regard to the depth to which float ing 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 for which is easily demonstrated; riz. As every body floating on a fluid will be immersed in proportion to its weight, and will displace a quantity of water equal thereto, it follows, that as a triangle is equal to only half a parallelogram of equal base and altitude, a parallelogram (or flat-bottomed 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 through the water than such as are more "bluff," or obtuse," at the bows:" the 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, thongle 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 à 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 ship 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, perceiving 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 necessary 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 HYDRAULICS 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

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an alkaline or earthy base. The general properties of these substances are, that they are soluble in water, and are chrystallizable; 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."

HYGROMETER, 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 water 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 contraction 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 employed 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 hygrometric quality more or less promptly; they had, therefore, the double inconvenience of be

ing inaccurate, and not being of long ser vice. 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 enveloped are immersed in a long-necked 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 vessels 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 proportion as the hair lengthens or shortens, it causes the cylinder to turn in one or the other direction, and by a necessary consequence, the little index turns likewise, the - motions of which are measured on the circumference of a graduated circle, about which the index performs its revolution as in common clocks. In this manner a very small variation in the length of the hair be comes perceptible, by the much more considerable motion that it occasions in the extremity of the index; and it will be easily conceived, that equal degrees of expansion, er of contraction in the hair, answer to equal 'VOL. III.

arcs described by the extremity of the in dex. To give to the scale such a basis as may establish a relation between all the hy grometers that are constructed upon the same principles, Saussure assumes two fixed terms, one of which is the extreme of humidity, and the other that of dryness: he determines the first by placing the hygrometer under a glass receiver, the whole interior surface of which he had completely moistened with water; the air being saturated by this water, acts by its humidity upon the hair to lengthen it. He moistened anew the interior of the receiver, as often as it was necessary; and he knew that the term of extreme humidity was attained, when, by a longer continuance under the receiver, the hair ceased to extend itself. To obtain the contrary limit of extreme dryness, the same philosopher made use of a hot and well-dried receiver, under which he included the hygrometer, with a piece of iron plate, likewise heated and covered with a fixed alkali. This salt, by exercising its absorbent faculty upon the remaining humidity in the surrounding air, causes the hair to contract itself, until it has attained the ultimate limit of its contraction. The scale of the instrument is divided into a hundred degrees. The zero indicates the limit of extreme dryness, and the number one hundred that of extreme humidity. The effects of moisture and of dryness upon the hair, are modified by those of heat, which act upon it, sometimes in the same sense, and sometimes in a contrary one; so that, if it be supposed, for example, that the air is heated about the hygrometer, on one part, this air, whose dissolving faculty with regard to the water will be augmented, will take away from the hair a portion of the water which it had imbibed, thus tending to shorten the hair; while, on the other part, the heat, by penetrating it, will tend, though much more feebly, to lengthen it ; and hence the total effect will be found to consist of two partial and contrary effects, the one hygrometric, the other pyrometric. In observations which require a certain precision, it is therefore necessary to consult the thermometer at the same time with the hygrometer; and on this account, the inventor has constructed, from observation, a table of correction, which will put it in the power of philosophers always to ascertain the degree of humidity of the air, from the effect produced by the heat.

De Luc, who devoted his attention to the same object, has followed a different Mu