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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 thesebodies, 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 •■ equilibrio; that whose affinity is the w eak est, will yield of its fluid to the other, iinriJ the equilibrium is established; and it is in this disposition of a body to moisten anothcr body that touches it, that what is called humidity properly consists. Of all bodies, the air is that of which we are me-; 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 bygronietric quality more or less promptly; they had, therefore, the double inconvenience of bt

irtg 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 Sausstire, 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 it hair, which Saussure first causes to undergo ■ 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 •f 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, Sanssure 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 tarn in one or the other direction, and by a necessary consequence, the little index turns likewise, the . motions of w hirb 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 becomes perceptible, by the much more considerable motion that it occasions in the extremity of the index; and it will Ire easily conceived, that equal degrees of expansion, •r of contraction in the hair, answer to equal VOL. III.

arcs described by the extremity of the in dexi To give to the scale such a basis as may establish a relation between all the hygrometers 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 pj rometiic. In observations which require a certain precision, it is therefore necessary to consult the thermometer at the same time with thu hygrometer. and ou 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 beat.

De Luc, who devoted Im attention to the same object, has followed a different Mm

method. This philosopher employed, for the construction of his hygrometers, a very thin slip of whale-bone, which performs the same office as the,hair in the hygrometer of Saussure. He kept this whale-bone bent by means of a spring, the action of which he preferred to that of a weight: he determined the degree of extreme humidity, by immersing the slip of whale-bone entirely under water; and to fix the opposite limit, which is that of extreme dryness, he made use of calcined lime, which he inclosed with the hygrometer under a glass bell. The choice of lime is founded on this, that the calcination having produced a higher degree of dryness, if it be afterward left to cool, so far that it may be placed without inconvenience under the glass bell destined for the experiment, it will be still found, as to sense, in the same state of dryness, since it is very slow in acquiring humidity; and thus all its absorbent faculty will be employed to dry up, by little and little, the air contained under the receiver, and to make the hygrometer itself pass to a state which approaches the nearest possible to extreme dryness. The hygrometer has been long neglected in meteorological observations; it is necessary to associate with it the thermometer and the barometer, to be in a state to unravel the complication of different causes which influence the variations of the atmosphere; and it is only by the aid of a long series of observations, made by these various instruments, together with all the indications which are deduced from the state of the heavens, that we can obtain such data as will enable us to prognosticate, with great probability, the temporary changes, and to arrive at a plausible theory upon this object, so interesting, and so naturally calculated to excite our curiosity.

Mr. Marshall says, that a simple instrument of this sort may be formed by means of " a flaxen line (large well-manufactured whipcord) five feet long; and having a graduated scale fixed to an index, moving on a fulcrum. The length of the index, from the fulcrum to the point, should be ten inches; that of the lever, from the fulcrum to the middle of the eye, to which the cord is fixed, two and a half." He adds, that "the principle on which this hygrometer acts is obvious. The air becoming moist, the cord imbibes its moisture; the line, in consequence, is shortened, and the index rises. On the contrary, the air becoming dry, the cord discharges its moisture,—

lengthens,—and the index falls. It may be true," he says," that no two hygrometers will keep pace with each other sufficiently to satisfy the curious. He will venture to say, however, from seven month's close attention, that two hygrometers, on this simple construction, have coincided sufficiently for the uses of agriculture. It is true," be adds, "they diminished in the degree of action; but as the scale may be readily diminished in extent, and as a fresh line may be so cheaply and so readily supplied, this is not a valid objection." It is remarked, that "tin's diminution, in the degree of action, depends considerably on the construction; the propriety, or rather delicacy, of which, rests, almost solely, on this point: the weight of the index should be so proportioned to the weight of the lever and cord, that the cord may be kept perfectly st raight, without being too much stretched. He made one with a long heavy index; and, in order to gain a more extensive scale, with a short lever; but, even when it was first put up,^ it could barely act; and, in a few weeks, it flagged, and was not able to raise the index, though the air was uncommonly moist. He therefore made another, Hud the same length, both of index and fever but with a lighter index, and a heavier lever so as to gain the proportion above-mentioned; and it has acted exceedingly well." He thinks that no farmer, "who wishes to profit by the hygrometer, should have lea than two. Three or four would be more advisable. They would then assist, in correcting each other; and, in case of renewal or alteration, there would be no danger of losing the state of the atmosphere; which, if only one is kept, must necessarily be the case. The principle on which this hygrometer is formed, is not, he says, confined to a small cord, and an index of ten inches long: it may be extended to a rope, of any length or thickness, and to an index and scale, of almost any dimensions and extent." But one, or more, ou a portable construction, might, he thinks, be found useful. An axe is the form he has thought of; the edge, graduated, will constitute the scale; and the handle will receive the cord: this may be. hung up, in the shade, exposed to the action of the air; or, by means of a spike in the end of the handle, it may be placed in the open field. By placing it on fallow ground, it may be actuated by the perspiration of the earth; among vegetables, by vegetable perspiration; by the means of one, or, more probably, by the

■leans of several placed at varied heights, -the different degrees of moisture at different altitudes may be ascertained, &c. , In tact, he considers the hygrometer, whether it is a prognostic of the weather or not, as a most valuable oracle to the farmer. See Weather.

HYMEN, in anatomy, a membrane sometimes of a circular, sometimes of a semilunar figure, and sometimes of a form difVerent from both.

HYMEN/EA, in botany, a genns of the Decandria Monogynia class and order. Natural order of Lomentacex. Leguminoss, Jussieu. Essential character: calyx fiveparted; petals five, almost equal; stylo twisted, inwards; legume filled with farinaceous pulp. There is only one species, viz. H. courbaril, locust-tree. The wild bees are fond of building their nests in this tree, which grows to a considerable size in the West Indies, and is looked upon as excellent timber; but it must be very old before it is cut, otherwise the heart will be but small. It is in great request for wheelwork in the sugar-mills, particularly for cogs to the wheels, being remarkably hard and tough. Professor Jacqnin says, that a cubic foot weighs about a hundred pounds, and that it will take a fine polish.

HYMENOPTERA, in natural history, the fifth order of insects according to the Ijnncvau system. The insects of this order are furnished with four membranaceous wings, and also with a sting, or a process resembling one. The wasp and the bee are insects of this order. It consists of the following genera:

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HYOSCYAMUS, in botany, kenbuni, a genus of the Pentaudria Monogynia class and order. Natural order of Lurid*. Solane*, Jussieu. Essential character: corolla funnel-form, obtuse; stamina inclined; capsule two-celled, covered with a lid. There arc eight species.

HYOSERIS, in botany, swine's Uttuce or tuccory, a genus of the Syngenesia Polygaraia iEqualis class and order. Natural order of Composite Semiflosculosi. Cichoracea?, Jussieu. Essential character: calyx almost equal ■ down hairy and calycled; receptacle naked. There are ten species.

HYPECOUM, in botany, a genus of the Tetrandria Digynia class and order. Natural order of Corydales. Papaveracex, Jussieu. Essential character: calyx twoleaved; petals fonr, the two outer broader, and trifid; fruit a silique. There are three species.

HYPELATE, in botany, a genus of the Polygamia Monoecia class and order. Essential character: calyx five-leaved; corolla five-pctalled; stigma bent down, threecornered ; drupe one-seeded. There is but one species, viz. H. trifoliata, a native of Jamaica, where it is common in the low lands.

HYPERBOLA, in geometry, the section, GE H, (Plate VII. Miscel. fig. 5.) of a cone, ABC, made by a plane, so that the axis, E F, of the section inclines to the opposite leg of the cone, BC, which in the parabola is parallel to it, and in the ellipsis intersects it. The axis of the hyperbolical section will meet also with the opposite side of the cone, when produced above the vertex, at D.

Definitions. 1. If at the point E (fig. 6.) in any plane, the end of the rule E H be so fixed, that it may be freely carried round, as about a centre; and at the other end of the rule H there is fixed the end of a thread shorter than the rule, and let the other end of the thread be fixed at the point F, in the same plane; bnt the distance of the points E F must be greater than the excess of the rule above the length of the thread; then let the thread be applied to the side of the rule E U, by the help of a pin G, and be stretched along it; afterwards let the rule be. carried round, and in the mean time let the thread kept stretched by the pin be constantly applied to the rule: a certain line will be described by the motion of the pin, which is called the hyperbola. But if the extremity of the same rule, which was Mm 2

method. This philosopher ©mp the construction of his hygromi thin slip of whale-bone, which p same office as the hair in the: of Saussure. He kept this « by MOM of a spring, the art he preferred to that of a wei mined the degree of cxtrenn immersing the slip of whale-! • under water; aud to fix tliwhich is that of extreme dry, use of calcined lime, win. the hygrometer under a choice of lime is founded on calcination having produced gree of dryness, if it be alt' cool, so far that it may inconvenience under the glass b for the experiment, it will b< to sense, in the same state of I it is very slow in acquiring I thus all its absorbent I ployed to dry np, by little air contained under th make the hygrometer il which approaches the D extreme dryness. The I been long neglected ob servations; it is Decesssrj i it the thermometer and be in a state to unravel different causes which in:'. tionsof the atmosp the aid of a long « made by these vari ther witli all the in duced from the st > we can obtain such di to prognosticate, with |

the temporary chang"

plausible theory upon

tercsting, and so nam,

cite our curiosity. Mr. Marshall sa>^

ment of this sort may

of "a flaxen line (hire

whipcord) five feet lot

duated scale fixed to at

fulcrum. The leiii.

the fulcrum to the pm

dies; that of the len

to the middle of the ej

is fixed, two and a hah

"the principle on wbi

acts is obvious. The air b

the cord imbibes it'

consequence, '* short'

rises. On the conti.

oiy, the cord discharges

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