Sauffure, as poffeffed of every property requifite for fuch an inftrument: as, 1. It points out the fmalleft variation of moisture in the air: 2. its indications are quick: 3. it is always confiftent: 4. feveral of them agree: 5. it is affected only by aqueous vapours: and 6. its variations are in proportion to thofe of the air.

rometer with the arbor. It may be augmented by making the fegment of the pulley to which the hair is fixed of a smaller diameter; but then the hair, in moving about it, would fret and contract stiffness, which would cause it to adhere to the bottom of the neck. M. Sauffure is of opinion, that the radius of this pulley should not be less than two lines, at least that there should be adapted a plate of filver or fome other contrivance; but then the hygrometer would be too difficult to conftruct, and it would require too much attention and care on the part of thofe who use it; his object was, to make an inftrument generally useful, easy, and convenient. The hygrometer with the arbor may be used for obfervations which require an extreme fenfibility. Both are accurately made by Mr Wm. Jones, Holborn, London. The variations of this inftrument, may be augmented by making it higher, because in that cafe longer hairs may be adapted: but it will be then lefs portable. Befides, if the hair is too long when obfervations are made in the open air, the wind communicates to the index inconvenient vibrations. It should not therefore exceed a foot in height. When it is of this dimenfion, a hair properly prepared can be applied to it, and its variations from extreme drynefs to extreme humidity are 80 or even 100 degrees; which on a a circle of 3 inches radius forms an extent fufficient for obfervations of this kind. M. Sauffure has even made smaller inftruments to be carried in the pocket, and to make experiments with under fmall receivers; they were only 7 inches high by two inches broad; which, notwithstanding their variations, werewery fenfible. In the preparation of the hair, it is neceffary to free it of its natural unctuofity, which in a great measure deprives it of its hygrometrical fenfibility. A number of hairs are boiled in a ley of vegetable alkali; and among thefe are to be chofen for ufe fuch as are moft transparent, bright, and foft: particular precautions are neceffary for preventing the ftraining of the hair, which renders it unfit for the intended purpose. The two fixed points of the hygrometer are the extremes both of moisture and drynefs. The former is obtained by expofing it to air completely faturated with water, by placing it in a glafs receiver standing in water, the fides of which are kept continually moistened. The point on the dial, at which the hand after a certain interval remains stationary, is marked 100. The point of extreme drynefs, not abfolute drynefs, for that does not exift, but the greatest degree of it that can be obtained, is produced by introducing repeatedly into the fame receiver containing the inftrument, and standing now upon quickfilver, certain quantities of deliquefcent alkaline falts, which abforb the moisture of the air. The highest point to which the hand can be brought by this operation, not only when it will rife no higher, but when it becomes retrograde from the dilatation occafioned by heat, is o; and the arch between these two points is divided into roo equal parts, or degrees of the hygrometer; the archpp, upon which the fcale is marked in the inftrument, being part of a circle of three inches diameter: hence every degree measures about

of a line. This hygrometer is confidered by M.

(6.) HYGROMETERS, Mr COVENTRY'S. The beft hygrometer upon the 3d principle, viz. that of the alteration of the weight of fubftances, by attracting the moisture of the air, and for afcertaining the quantity as well as the degree of moif. ture in the variation of the hygrometer, is that of Mr Coventry of Southwark, London. The account he gives of it is as follows: "Take two fheets of fine tiffue paper, fuch as is used by hatters; dry them carefully at about two feet distance from a tolerably good fire, till after repeatedly weighing them in a good pair of fcales no moisture remains. When the theets are in this perfectly dry state, reduce them to exactly 50 grains; the hygrometer is then fit for ufe. The fheets must be kept free from duft, and expofed a few minutes in the open air; after which may be always known, by weighing them, the exact quantity of moisture they have imbibed. For many years (adds he) the hygrometer has engroffed a confiderable share of my attention; and every advantage propofed by others, either as it refpected the fubftances of which the inftrument was compofed, or the manner in which its operations were to be difcerned, has been impartially examined. But I have never feen an hygrometer fo fimple in itself, or that would act with fuch certainty or fo equally alike, as the one I have now defcribed. The materials of which it is compofed being thin, are easily deprived wholly of their moifture; which is a circumftance effentially necessary in fixing a datum from which to reckon, and which, I think, cannot be said of any fubftance hitherto employed in the conftruction of hygrometers: with equal facility they imbibe or impart the humidity of the atmosphere, and fhow with the greateft exactness when the least alteration takes place." For eafier weighing the paper, take a piece of round tin or brafs the fize of a crown-piece, through the centre of which drill a hole, and alfo three others round it at equal diftances: then cut about 100 papers; and after putting them under the tin or brafs, drive through each hole a ftrong pin into a board, in order to round them to the fhape of the plate: the papers muft then be separated and exposed to the air a few hours with that already weighed, and so many of them taken as are equal to the weight already specified. This done, thread them together through thefe holes made by the pins, putting between every paper on each thread a small bead, to prevent the papers from touching each other, and alfo that the air may be more readily admitted. The top of the hygrometer is covered with a card cut to the fame fize; and which, by its stiffness, fupports all the papers, and keeps them in proper fhape. Before the papers are threaded, the beads, filk, card, and a thin piece of brafs about the fize of a fixpence, which must be placed at the bottom, and through which the centre ftring passes, muft be weighed with the greateft exactness, to bring them to a certain weight, suppose 50 grains; now

the

observing the almoft perpetual alteration of the atmosphere, even in the moft fettled weather. In winter it will be conftantly traverfing from about 8 A. M. till 4 or 5 P. M. towards dry; and in fummer, from about 4 A. M. till 6 or 7 P. M. when the weather is hot and gloomy, the hygrometer difcovers a very great change towards moisture; and when clear and frofty, that it contains a much greater quantity of moisture than is generally imagined."

HYGROMETRICAL. adj. belonging to a hy

grometer.

HYGROMETRY. n. f. the art or science of measuring the moisture of the atmosphere. See HYGROMETER, § 1—6.

*

(1.) HYGROSCOPE. n. f. [vye and exore; hygroscope, Fr.] An inftrument to fhew the moifture and drynefs of the air, and to measure and eftimate the quantity of either extreme. QuincyMoisture in the air is discovered by hygroscopes. Arbuthnot...

(2.) HYGROSCOPE is commonly used in the fame fenfe with HYGROMETER, but Wolfius makes a difference from the etymology of the words. The hygrofcope (he fays) only fhews the changes of humidity or drynefs in the air, but the hygrometer measures them.

HYGROSCOPICAL. adj. fhewing the moif. ture or dryness of the atmosphere.

HYLA, in ancient geography, a river of Mysia Minor, famous for the death of HYLAS. It runs by Prufa, whence it feems to be the fame with the RYNDACUS, which runs NW. into the Propontis.

the paper in its driest state being of equal weight, they will weigh together 100 grains; confequently what they weigh more at any time is moisture. To obviate the difficulty of trying experiments with weights and scales, Mr Coventry contrived a machine or scale by which to determine at one view the humidity or drynefs of the atmosphere. This, with its cafe, is reprefented by fig. 3, pl. CXCI. The front and back of the cafe are glafs; the fides fine gauze, which excludes the duft and admits the air; the cafe is about 10 inches high, 8 inches broad, and 4 inches deep. A, a brass bracket in front, behind which, at about 3 inches distance, is another; these support the axis of the index E, alfo of the beam D, and another which fupports the ftem B, to which the ivory scale of divifions C is fixed G, a brafs fcale fufpended in the ufual manner to the end of a beam D, and weighing exactly 100 grains. This fcale is an exact counterpoife to the papers I and the different apparatus. The manner of fufpenfion is as follows: The axis of the beam g, which is made of brass, instead of hanging on pivots as in common scales, turns with two fteel edges kk, fixed in the extremities of the brafs axis: thefe edges are fhaped like the edge of a knife, and act on two fteel concave edges 11, in order to render the friction as fmall as poffible. D is a fine fcale beam fixed at right angles with the axis g. E, the fteel. index fixed to the under fide of the fame axis. F, a brafs fliding weight: b is the axis that holds the ftem, B to which the scale of divifions C is fixed. AA, the brafs brackets which fupport the whole by four fcrews, two of which are feen at ii, that fcrew the brackets to the top of the cafe. The axis of the scale of divifions is hung on pivots, one of which is feen at m, that, fhould the cafe not ftand level, the ftem B may always be in a perpendicular fituation. The hygrometer, before ufe, fhould be thus adjusted: To the end of the beam where the hygrometer is fufpended, hang a weight of 100 grains, which is equal to the weight of the fcale; then move the fliding weight F up or down the index E, till one grain will caufe the index to traverse neither more nor less than the whole scale of divifions; then add half a grain to the fcale, in order to bring the index to o; and the inftrument, after taking off the 100 grain weight and hanging on the papers, is fit for ufe; then put grain weights in the fcale till the index is brought within compafs of the fcale of divifions., Example: H is 3 grains on the brafs, fcale, and the index points at 10; confequently there is 3 grains and 10 hun dredths of a grain of moisture in the papers. If four grain weights are kept, viz. 1, 2, 4, and 5, they will make any number from 1 to 9, which are as many as will be wanted. Sometimes the index will continue traverfing within the fcale of divifions for many days without fhifting. the weights; but if otherwife, they must be changed as occafion may require. "One great advantage (fays Mr Coventry) of this hygrometer, above all others that have attracted my, notice, is that it acts from a certain detum, namely, the dry extreme; from which all the variations towards moift are calculated with certainty and if conftructed with that precifion reprefented by the drawing, it will afford pleasure to the curious in

* HYLARCHICAL. adj. [an and agx".] Prefiding over matter.

HYLAS, in fabulous hiftory, fon of Theodamus, and favourite of Hercules. He was ravifhed by the nymphs of a fountain as he was taking out water; and afterwards drowned in the HYLA.

HYLE, a lake of Chefh. between Dee and Mersey.

"

HYLL, Alban, or Albayn, M. D. a phyfician of the 16th century, a native of Scotland, or, according to others, of Wales. He ftudied at Oxford, but graduated abroad. He wrote commentaries on Galen's works; and died at London in 1559.

HYLLUS, in fabulous hiftory, the son of Hercules and Dejanira. See HERACLIDE.

HYLOGONES. See ETHIOPIA, 3. HYLOPATHIANS, ancient Greek philofophers, who derived all things from dead and ftupid matter, in the way of qualities and forms, generable and corruptible. See ANAXIMANDRIANS, and Cudworth's Intellectual System, B. i. ch. 3.

HYLOZOISTS, ['van, matter, and (w», life,] a fect of atheists among the ancient Greek philofophers, who maintained that matter had fome natural perception, without animal fenfation, or reflection in itself confidered; but that this imperfect life occafioned that organization, whence fenfation and reflection afterwards arofe. Of these, fome held only one life, which they called a PLASTIC nature, prefiding regularly and invariably over the whole corporeal univerfe, which they reprefented as a kind of large plant or vegetable: these were called the cofmoplaftic and ftoical atheifts, becaufe the Stoics held fuch a nature,

though