Imágenes de páginas
PDF

of the case, and commit themselves to the water.

HYDROPHOBIA, in medicine, an aversion or dread of water; a terrible symptom of the rabies canini. See Medicine.

HYDROPHYLAX, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Rubiacec, Jussieu. Essential character: calyx four-parted; corolla funnel-form ; fruit ancipital, one-seeded. There is only one species, viz. H. mirilima, found in driving-sand, on the seashore, near Guduluhr in the East Indies.

HYDROPHYLLUM, in botany, irutertfaf, a genus of the Peutandria Monogynia class and order. Natural order of Boragima\ Jnssieu. Essential character: corolla bell-shaped, having five longitudinal melliferous streaks on the inside; stigma bifid ; capsule globular, two-valved. There are two species, riz. H. virginicum, Virginian water-leaf; and H. canadense, Canadian water-leaf.

HYDROSCOPE, an instrument anciently used for the measuring of time. The hydroscope was a kind of water-clock, consisting of a cylindrical tube, conical at bottom: the cylinder was graduated, or marked out with divisions, to which the top of the water, becoming successively contiguous, as it trickled out at the vertex of the cone, pointed out the hour.

HYDROSTATICAL balance, a kind of balance contrived for the easy and exact finding the specific gravities of bodies, both liquid and solid. See Hydrostatics.

HYDROSTATICS relate to the resting equilibrium of non elastic fluids; and to the pressure of solids immersed therein. A fluid is a body whose parts are infinitely minute, capable of dislocation in consequence of the smallest force, invariably, (when suffered to rest), resuming a perfect level surface, and presenting an equal resistance, throughout every part to the body immersed.

Philosophers consider fluids to be divided into two classes, riz. the elastic, such as air, vapour, and gas; all which may be compressed more or less: and the inelastic, Ariz. water, mercury, spirits, &c, which cannot be compressed; though by being heated they distend considerably. It may be proper to observe in this place, that Mr. Canton in the years 1762 and 1764, published the results of experiments he had made, whereby it was endeavoured to be proved, that all fluids were compressible though in so trifling a degree as not to affect their bulks when under examination. With the barometer at 21'J, and the thermometer at 50, he declares the following compressions were effected.

Spec. Griiv. Compression

With Spirit of wine 846 66 parts in a million

Oil of olives 918 48 ditto

Rain water 1,000 46 ditto

Sea water 1,028 40 ditto

Mercury 13,595 3 ditto

We leave the reader to judge whether it be probable by any apparatus of human formation, and under human guidance, to ascertain that the three millionth parts, said, to have beep compressed, were really so. Indeed, even the sixty-sixth millionth parts, suffered to be compressed in the spirits of wine, must appear extremely doubtful ; though we cannot but conclude that, as air exists in every atom of nature, more or less, with a sufficient force every fluid were subject to compression into a smaller space than is occupied by it when perfectly at liberty. Speaking generally, the definitions above given may be considered as applicable to all cases with which we are acquainted; and may, perhaps, be completely true.

We shall commence the detail, incident to this subject, with an account of the method of obtaining the specific gravities of bodies: that is, by shewing the comparative weights of various solids, and fluids, as ascertained by the most careful and skilful chemists. The reader must, however, consider the weights as taken at a medium. See Gravity, specific, where is given a table of specific gravities.

The reader will observe, that the whole of the above are compared with rain-water, which appear at 1,000 parts; but it is very remarkable that the density of that fluid varies greatly according to its temperature , and that it by no means affords a regular scale of weight, or of bulk in proportion to the degrees of heat. This will be seen

[merged small][table]

We must suppose the water of the Dead Sea to be highly impregnated; since it appears to weigh nearly a fourth more than common sea water.'

The anomalies lay between 32° and 45% and are accounted for by the contraction which takes place in water about to freeze, and its sudden expansion afterwards: by this we understand the course of bottles, pitchers, &c. being burst when the water they contain freezes. The difference in bulk between water and liquors in the winter, and in the summer season, averages about three per cent: hence many great dealers have thought it worth their while to buy only in the former season, when the liquors have been most concentrated.

The specific gravity of a body cither fluid, or solid, is ordinarily found by means of the hydrostatic balance; a most ingenious device for exactly ascertaining the 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 horsehair, 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 y*lmich 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 7$ 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 weight 7 j arrair*. 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 1 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 equilibrium in the water, pour in the quicksilver, and the additional weight requisite to counterbalance it will show its exact weight.

Perhaps the following general rules for finding the specific gravity of bodies may prove useful and familiar to every understanding. 1. " When the body is heavier than water." Weigh it both in water, and in the atmosphere, and the difference between the results will shew 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 ah,

so is the gravity of water to the gravity of the body. S. " When a fluid is to be neighed." Weigh the fluid in a cup, which is to be deemed an appendage^ and treated according to the foregoing rule, observing, that as the v. hole 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; (ri:. 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 he 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 way alike, though their general tendency is to gravition. 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 reprises it. But wb«n the finger is with

drawn, 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. Thus, in a pan whose 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 oppressed as much as if the sides were cylindrical. 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 V, 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. We have indeed further proof of 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 disproportionately heavy, and will admit the water between the top and bottom 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 lift and support three weights, of 100 lb. each. Where either airor any other fluid is debarred from access between two planks 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 exciting their prcssurc,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 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. It 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; it 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 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 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 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 diting-bellr 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

open atmosphere which would fill the interior of the tumbler. If the inverted tumbler wire 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.

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 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, 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 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 titan 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 tulphuret, in chemistry, the combination of sulphuretted hydrogen, with

« AnteriorContinuar »