an instrument for measuring the degrees of dryness or moisture of the atmosphere, in like manner as the barometer and thermometer measure its different degrees of gravity or warmth.

Though every substance which swells in moist, and shrinks in dry weather, is capable of becoming an hygrometer; yet this kind of instrument is far from being as yet arrived at such a degree of perfection as the barometers and thermometers. There are three general principles on which hygrometers have been constructed.

1. The lengthening and shortening of strings by dryness and moisture, or their twisting and untwisting by the same.

2. The swelling and shrinking of solid substances by moisture or dryness; and,

3. By the increase or decrease of the weight of particular bodies whose nature is to absorb the humidity of the atmosphere.

On the first of these principles Mr Smeaton constructed an hygrometer greatly superior to any that had appeared before; and of which the following account is given in the 62nd volume of the Philosophical Transactions.

Having some years ago attempted to make an accurate and sensible hygrometer by means of a hempen Hygrometer cord of a considerable length, I quickly found, that though it was more than sufficiently susceptible of every change in the humidity of the atmosphere, yet the cord was upon the whole in a continual state of lengthening. Though this change was the greatest at first, yet it did not appear probable that any given time would bring it to a certainty; and furthermore, it seemed, that as the cord grew more determinate in mean length, the alteration by certain differences of moisture grew less. Now, as on considering wood, catgut, paper, &c., there did not appear to be a likelihood of finding any substance sufficiently sensible of differences of moisture that would be unalterable under the same degrees thereof; this led me to consider of a construction which would readily admit of an adjustment; so that, though the cord whereby the instrument is actuated may be variable in itself, both as to absolute length, and difference of length under given degrees of moisture, yet that, on supposition of a material departure from its original scale, it might be readily restored thereto; and, in consequence, that any number of hygrometers, similarly constructed, might, like thermometers, be capable of speaking the same language.

The two points of heat the more readily determinable in a thermometer, are the points of freezing and boiling water. In like manner, to construct hygrometers which shall be capable of agreement, it is necessary to establish two different degrees of a moisture which shall be as fixed in themselves, and to which we can have recourse as readily and as often as possible.

One point is given by making the substance perfectly wet, which seems sufficiently determinable; the other is that of perfect dry, which I do not apprehend to be attainable with the same precision. A readiness to imbibe wet, so that the substance may be soon and fully saturated, and also a facility of parting with its moisture on being exposed to the fire to dry, at the same time, that neither immersion, nor a moderate exposition to the warmth of the fire, shall injure its texture, are properties requisite to the first mover of such an hygrometer, that in a manner exclude all substances that I am acquainted with, besides hempen and flaxen threads and cords, or substances compounded of them.

Upon these ideas, in the year 1758, I constructed two hygrometers as nearly alike as possible, in order that I might have the means of examining their agreement or disagreement on similar or dissimilar treatment. The interval or scale between dry and wet I divided into 100 equal parts, which I call the degrees of this hygrometer. The point of 0 denotes perfect dry; and the numbers increase with the degrees of moisture to 100, which denotes perfect wet.

On comparing them for some time, when hung up together in a passage or stair-case, where they would be very little affected by fire, and where they would be exposed to as free an air as possible in the inside of the house, I found that they were generally within one degree, and very rarely differed two degrees; but as these comparisons necessarily took up some time, and were frequently interrupted by long avocations from home, it was some years before I could form a tolerable judgment of them. One thing I soon observed, not altogether altogether to my liking, which was, that the flaxen cords made use of seemed to make so much resistance to the entry of small degrees of moisture (such as is commonly experienced within doors in the situation above mentioned), that all the changes were comprised within the first 3° of the scale; but yet, on exposing them to the warm steam of a wash-house, the index quickly mounted to 100. I was therefore desirous of impregnating the cords with something of a saline nature, which should dispose them more forcibly to attract moisture; in order that the index might, with the ordinary changes of the moisture in the atmosphere, travel over a greater part of the scale of 100. How to do this in a regular and fixed quantity, was the subject of many experiments and several years interrupted inquiry.

At last I tried the one hereafter described, which seemed to answer my intention in a great measure; and though upon the whole it does not appear probable that ever this instrument will be made capable of such an accurate agreement as the mercurial thermometers are, yet if we can reduce all the disagreements of an hygrometer within 1/25th part of the whole scale, it will probably be of use in some philosophical inquiries, in lieu of instruments which have not yet been reduced to any common scale at all.

"Fig. 1. and 2. ABC is an orthographic delineation of the whole instrument seen in front in its true proportion. DE is that of the profile, or instrument seen edgewise. FG in both represents a flaxen cord about 35 inches long, suspended by a turning peg F, and attached to a loop of brafs wire at A, which goes down into the box cover H, and defends the index, &c. from injury; and by a glass exposes the scale to view.

"Fig. 3. shows the instrument to a larger scale, the upright part being shortened, and the box-cover removed; in which the same letters represent the same parts as in the preceding figures; GI are two loops or long links of brafs wire, which lay hold of the index KL, moveable upon a small stud or centre K. The cord FG is kept moderately strained by a weight M of about half a pound avoirdupois.—It is obvious, that, as the cord lengthens and shortens, the extreme end of the index rises and falls, and successively passes over N 2 the scale disposed in the arch of a circle, and containing 100 equal divisions. This scale is attached to the brafs sliding ruler QP, which moves upon the directing piece RR, fixed by screws to the board, which makes the frame or base of the whole; and the scale and ruler NOP is retained in any place nearer to or further from the centre K, as may be required by the screw S.

"Fig. 4. represents in profile the sliding piece and stud I (fig. 3.), which traverses upon that part of the index next the centre K; and which can, by the two screws of the stud, be retained upon any part of the index that is made parallel; and which is done for three or four inches from the centre, for that purpose. The stud is filed to the edges, like the fulcrum of a scale-beam; one being formed on the under side, the other on the upper, and as near as may be to one another. A hook formed at the lower end of the wire-loops CI, retains the index, by the lowermost edge of the stud; while the weight M hangs by a small hook upon the upper edge: by these means the index is kept steady and the cords strained by the weight, Hygrometer, with very little friction or burden upon the central stud K.

"Fig. 5. is a parallelogram of plate-brafs, to keep Fig. 5. out of sight, which is attached to the upper edge of the box-cover H; and serves to shut the part of the box-cover necessarily cut away, to give leave for the wire GI to traverse with the sliding stud nearer to or further from the centre of the index K; and where, in fig. 5. a is a hole of about an inch diameter, for the wire GI to pass through in the rising and falling of the index freely without touching; b is a slit of a lesser size, sufficient to pass the wire, and admit the cover to come off without deranging the cord or index; cc are two small screws applied to two slits, by which the plate slides lengthwise, in order to adapt the hole c to the wire GI, at any place of the stud I upon the index KL.

"1. In this construction, the index KL being 12 inches long, 4 inches from the extreme end are filed so narrow in the direction in which it is seen by the eye, that any part of these four inches lying over the divisions of the scale, becomes an index thereto. The scale itself slides four inches, so as to be brought under any part of the four inches of the index attenuated as above mentioned.

"2. The position of the directing piece RR is so determined as to be parallel to a right line drawn through o upon the scale, and the centre K of the index; consequently, as the attenuated part of the index forms a part of a radius or right line from the same centre, it follows, that whenever the index points to o upon the scale, though the scale is moved nearer to or further from the centre of the index, yet it produces no change in the place to which the index points.

"When the divided arch of the scale is at 10 inches from the centre (that is, at its mean distance); then the centre of the arch and the centre of the index are coincident. At other distances, the extremes of which are eight or twelve inches, the centre of the divisions, and the centre of the index pointing thereto, not being coincident, the index cannot move over the spaces geometrically proportionable to one another in all situations of the scale; yet the whole scale not exceeding 32° of a circle, it will be found on computation, that the error can never be so great as 1/60th part of the scale, or 1° of the hygrometer; which in this instrument being considered as indivisible, the mechanical error will not be sensible.

"The cord here made use of is flax, and between 1/50th and 1/75th of an inch in diameter; which can be readily ascertained by measuring a number of turns made round a pencil or small stick. It is a sort of cord used in London for making nets, and is of that particular kind called by net-makers flaxen three-threads laid. A competent quantity of this cord was boiled in one pound avoirdupois of water, in which was put two pennyweights troy of common salt; the whole was reduced by boiling to six ounces avoirdupois, which was done in about half an hour. As this ascertains a given strength of the brine, on taking out the cord, it may be supposed that every fibre of the cord is equally impregnated with salt. The cord being dried, it will be proper to stretch it; which may be done so as to prevent it from untwisting, by tying... three or four yards to two nails against a wall, in an horizontal position, and hanging a weight of a pound or two to the middle, so as to make it form an obtuse angle. This done for a week or more in a room, will lay the fibres of the cord close together, and prevent its stretching so fast after being applied to the instrument as it would otherwise be apt to do.

"The hygrometer is to be adjusted in the following manner. The box-cover being taken off to prevent its being spoiled by the fire, and choosing a day naturally dry, set the instrument nearly upright, about a yard from a moderate fire; so that the cord may become dry, and the instrument warm, but not so near as would spoil the finest linen by too much heat, and yet fully evaporate the moisture; then let the instrument stay till the index is got as low as it will go; now and then stroking the cord betwixt the thumb and finger downwards, in order to lay the fibres thereof close together; and thereby causing it to lengthen as much as possible. When the index is thus become stationary, which will generally happen in about an hour, more or less as the air is naturally more or less dry, by means of the peg at top raise or depress the index, till it lies over the point o. This done, remove the instrument from the fire; and having ready some warm water in a tea-cup, take a middling camel's hair pencil, and dipping it in the water, gently anoint the cord till it will drink up no more, and till the index becomes stationary and water will have no more effect upon it, which will also generally happen in about an hour. If in this state the index lies over the degree marked 100, all is right: if not, slack the screw S, and slide the scale nearer to or further from the centre, till the point 100 comes under the index, and then the instrument is adjusted for use: but if the compass of the slide is not sufficient to effect this, as may probably happen on the first adjustment, slack the proper screws, and move the sliding stud I nearer to or further from the centre of the index, according as the angle formed by the index between the two points of dry or wet happens to be too small or too large for the scale."

On this principle, a simple hygrometer has been made by Mr Coventry of Southwark, London. It is not upon the most accurate construction, yet will act very sensibly in the common changes of the air. Fig. 6 represents the hygrometer as applied to a wall or board. A is a string of whip-cord, catgut, &c. of any length at pleasure: it is suspended on a bracket B, and kept extended by a weight at the bottom C. DD is a slip of wood, which with the bracket is fixed perpendicularly to a wall or side of a room. It has a straight line E drawn down in the middle of the board, serving to point out the divisions upon the edges of the two thin circular cards F and G. At the centre of the bottom of each of these cards is glued a piece of cork, through which the string A is drawn: These cork pieces serve to preserve the horizontal position of the cards. The upper card F is divided into 10 equal parts or divisions, and the under card G into 100 equal parts; the string A being measured into 10 equal parts, from the point of suspension H to the surface of the lower card I. The card F is hung at the first part, from H, and the card G at the 10th part from the same point; consequently, from the twisting and untwisting of the string A by the different changes of the air, the lower card G, from the mechanical principles of motion, will describe 10 revolutions for one of the upper card F; or when the lower card G has made one revolution, the upper card F will have described but the 10th part, or one of its divisions. From whence it appears, that by the assistance of the upper card F, an index is thereby obtained of the number of revolutions the lower card G performs, which are reckoned by the line E on the slip of wood.

Example. It must first be observed what division of the card F the line is against, suppose 3; and also what division of the lower card G is cut by the same line, suppose 10: it then appears, that the state of the hygrometer is thus, 3 degrees and 10 hundredths of another. If the whole 10 divisions of the card have passed the line E, the lower card G will have revolved 10 times, or 10 hundred parts, equal to 1000; the accuracy to which the principle of this simple contrivance answers. Before use, the hygrometer should be adjusted; to do which, the cards F and G are first set to the line E at the 0 of each, or commencement of the graduations: whatever direction the cards afterwards take, it must evidently be from the change to greater moisture or dryness in the air; and they will accordingly point it out.

On this principle, but with a degree of ingenuity Saussure has pains perhaps never before employed, an hygrometer has been constructed by M. de Saussure, professor of philosophy at Geneva. In his Essais sur l'Hygrométrie, in 4to, 1783, is an important detail on the subject of hygrometry; from which the following description of his hygrometer is taken. The author found by repeated experiments, that the difference between the greatest extension and contraction of a hair, properly prepared, and having a weight of about three grains suspended to it, is nearly $\frac{1}{10}$ of its whole length; that is, $3\frac{3}{4}$ or $3\frac{3}{4}$ lines in a foot. This circumstance suggested the idea of a new hygrometer: and, in order to render those small variations perceptible and useful, the following apparatus was constructed.

Fig. 7. is a representation of the whole instrument, Fig. 7. with the hair and other appendages complete. The lower extremity of the hair ab is held by the chaps of the screw pincers b. These pincers are represented aside at B: by a screw at its end, it fastens into the nut of the bottom plate C. This nut of the plate turns independently of the piece that supports it, and serves to raise or depress the pincers B at pleasure.

The upper extremity a of the hair is held by the under chaps of the double pincers a, represented aside at A. These pincers fasten the hair below, and above fasten a very fine narrow slip of silver, carefully annealed, which rolls round the arbor or cylinder d, a separate figure of which is shown at DF. This arbor, which carries the needle or index ee, or E in the separate figure, is cut into the shape of a screw; and the intervals of the threads of this screw have their bases flat, and are cut squarely so as to receive the slip of silver that is fastened to the pincers a, and joined in this manner with the hair. M. Saussure observes, that hair alone fixed immediately to the arbor would not do; for it curled upon it, and acquired a stiffness that the counterpoise was not able to surmount. The arbor was cut in a screw form, in order that the slip of silver in wind- ing upon it should not increase the diameter of the arbor, and never take a situation too oblique and variable. The flip is fixed to the arbor by a small pin F. The other extremity of the arbor D is shaped like a pulley, flat at the bottom so as to receive a fine supple silken string, to which is suspended the counterpoise g in the large figure, and G in the side one. This counterpoise is applied to defend the hair; and acts in a contrary direction to that of the hair, and the moveable pincers to which the hair is fixed. If then the hair should be loaded with the weight of four grains, the counterpoise must weigh four grains more than the pincers. The arbor at one end passes through the centre of the dial, and turns therein, in a very fine hole, on a pivot made very cylindrical and well polished: at the other end is also a similar pivot, which turns in a hole made in the end of the arm h of the cock h i, HI. This cock is fixed behind the dial by means of the screw I.

The dial k e e k, divided into 360 degrees, is supported by two arms l l; these are foldered to two tubes, which inclose the cylindrical columns m m m m. The setting screws n n move upon these tubes, and serve thereby to fix the dial and arbor to any height required. The two columns which support the dial are firmly fastened to the case of the hygrometer, which rests upon the four screws o o o o; by the assistance of these screws, the instrument is adjusted, and placed in a vertical situation.

The square column p p, which rests upon the base of the hygrometer, carries a box q, to which is fixed a kind of port-crayon r, the aperture of which is equal to the diameter of the counterpoise g. When the hygrometer is to be moved from one place to another; to prevent a derangement of the instruments from the oscillations of the counterpoise, the box q and the port-crayon r must be raised up so as the counterpoise may fall into and be fixed in it, by tightening the screw s and the box and counterpoise together by the screw t. When the hygrometer is intended for use, the counterpoise must be disengaged by lowering the box, as may be conceived from the figure.

Lastly, at the top of the instrument is a curved piece of metal x y z, which is fastened to the three columns just described and keeps them together. It has a square hole at y, which serves to hang up the hygrometer by when required.

The variations of which this hygrometer is capable, are (all things besides equal) as much greater as the arbor round which the flip of silver winds is than a smaller diameter, and as the instrument is capable of receiving a longer hair. M. Saufure has had hygrometers made with hairs 14 inches long, but he finds one foot sufficient. The arbor is three-fourths of a line in diameter at the base between the threads of the screw or the part on which the flip winds. The variations, when a hair properly prepared is applied to it, are more than an entire circumference, the index describing about 400 degrees in moving from extreme dryness to extreme humidity. M. Saufure mentions an inconvenience attending this hygrometer, viz. its not returning to the same point when moved from one place to another; because the weight of three grains that keeps the silver flip extended, cannot play to exactly as to act always with the same precision against the arbor round which it winds. But this weight cannot be sensibly increased without still greater inconveniences: he therefore observes, that this hygrometer is well calculated for a fixed situation in an observatory, and for various hygrometrical experiments; since, instead of the hair, there may be substituted any other substance of which a trial may be wanted; and it may be kept extended by a counterpoise more or less heavy as they may require: but the instrument will not admit of being moved, nor serve even for experiments which may subject it to agitation.

To obviate the objection above mentioned, M. Saufure has contrived another apparatus more portable, hygrometer and convenient, and which, if not so extensive in its variations, is in fact very firm, and not in the least liable to be deranged by carriage and agitation. Fig. 8. is a Fig. 8. representation of this hygrometer, which he calls the portable hygrometer, in distinction from the preceding, which he calls the great hygrometer or the hygrometer with the arbor. The material part of this instrument is its index a b c e; an horizontal view of which, and the arm that carries it, is seen in the separate figure GBDEF. This index carries in its centre D a thin tube hollow throughout, and projects out on each side of the needle. The axis which passes through it, and round which the index turns, is made thin in the middle of its length and thick at the ends; so that the cylindrical tube which it passes through touches it only at two points, and acts upon it only at its extremities.

The part d E of the index serves to point out and mark on the dial the degrees of moisture and dryness; the opposite part d b DB serves to fix both the hair and counterpoise. This part which terminates in a portion of a circle, and is about a line in thickness, is cut on its edge in a double vertical groove, which makes this part similar to the segment of a pulley with a double neck. These two grooves, which are portions of a circle of two lines radius, and have the same centre with that of the index d, serve in one of them to contain the hair, and in the other the silk, to the end of which the counterpoise is suspended. The same index carries vertically above and below its centre two small screw-pincers, situated opposite to the two grooves: that above at a, opposite to the hindmost groove, serves to fix the silk to which the counterpoise is suspended; and that below at b, opposite to the highestmost groove, serves to hold one of the ends of the hair. Each of these grooves has its partitions cut, as seen in the section B, and its bottom made flat in order that the hair and silk may have the greatest freedom possible. The axis of the needle DD goes through the arm g f GF, and it is fixed to this arm by the tightening screw f F. All the parts of the index should be in perfect equilibrium about its centre; so that when it is on its pivot without the counterpoise, it will rest indifferently in any position it may be placed in.

It must be understood, that when the hair is fixed by one of its extremities in the pincers e, and by the other end on the pincers y at the top of the instrument, it passes in one of the necks of the double pulley b, whilst the counterpoise to which the silk is fixed in a passes in the other neck of the same pulley: the counterpoise serves to keep the hair extended, and acts always in the same direction and with the same force, whatever. whatever the situation of the index may be. When therefore the dryness contracts the hair, it overpowers the gravity of the counterpoise, and the index descends; when, on the contrary, the humidity relaxes the hair, it gives way to the counterpoise, and the index ascends. The counterpoise should weigh but three grains; so that the index should be made very light and very easy in its motion, in order that the least possible force may move it and bring it back again to its point when drawn aside.

The dial \( h \) is a circular arch, the centre of which is the same with that of the index. This arch is divided into degrees of the same circle, or into the hundredths of the interval which is found between the limits of extreme dryness and extreme humidity. The interior edge of the dial carries at the distance \( h \) a kind of projecting bridle or stay \( i \), made of brass wire, curved to the arch, and fixed in the points \( i \). This bridle retains and guards the index, at the same time leaving it to play with the requisite freedom. The screw-pincers \( y \), in which is fastened the upper extremity of the hair, is carried by a moveable arm, which ascends and descends at pleasure the length of the frame \( K \). This frame is cylindrical everywhere else, except its being here flattened at the hinder part to about half its thickness, in order that the piece with the screw which carries the arm should not project out underneath, and that the arm may not turn. The arm may be stopped at any desired height by means of the pressing screw \( x \). But as it is of use sometimes to be able to give the instrument a very small and accurate motion, so as to bring the index exactly to the part that may be wanted, the slide piece \( l \), which carries the pincers \( y \), to which the hair is fixed, is to be moved by the adjusting screw \( m \).

At the base of the instrument is a great lever \( n o p q \), which serves to fix the index and its counterpoise when the hygrometer is to be moved. The lever turns an axis \( n \), terminated by a screw which goes into the frame; in tightening this screw, the lever is fixed in the desired position. When the motion of the index is to be stopped, the intended position is given to this lever, as represented in the dotted lines of the figure. The long neck \( p \) of the lever lays hold of the double pulley \( b \) of the index, and the short neck \( o \) of the counterpoise: the tightening screw \( q \) fastens the two necks at once. In confining the index, it must be so placed, that the hair be very slack; so that, if whilst it is moved the hair should get dry, it may have room to contract itself. Afterwards, when the instrument is placed for use, the first thing to be done is to relax the screw \( n \), and turn back the double lever with great caution, taking equal care at the same time not to strain the hair. It is better to apply one hand to the index near its centre, whilst the other hand is disengaging the pulley and the counterpoise from the lever that holds them steady. The hook \( r \) serves to suspend a thermometer upon; it should be a mercurial one, with a very small naked bulb or ball, so as to show in the most sensible manner the changes of the air: it should be mounted in metal, and guarded in such a manner as not to vibrate so as to break the hair. Lastly a notch is made under the top of the frame \( s \), to mark the point of suspension, about which the instrument is in equilibrium, and keeps a vertical situation.

All the instrument should be made of brass: though Hygrometer's axis of the index and its tube work more pleasantly together if made of bell-metal.

The extent of this hygrometer's variations is not more than the fourth or fifth part of the hygrometer with the arbor. It may be augmented by making the segment 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 a stiffness, which would cause it to adhere to the bottom of the neck. M. Sauflure 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 silver or some other contrivance; but then the hygrometer would be too difficult to construct, and it would require too much attention and care on the part of those who use it: his object was, to make an instrument generally useful, and easy and convenient in its use. The hygrometer with the arbor may be used for observations which require an extreme sensibility.

The variations of this instrument may be augmented by making it higher, because in that case longer hairs might be adapted: but it would be then less portable. Besides, if the hair is too long, when observations are made in the open air, the wind has too great an effect upon it, and thus communicates to the index inconvenient vibrations. It is not proper therefore to make it more than a foot in height. When it is of this dimension, an hair properly prepared can be applied to it, and its variations from extreme dryness to extreme humidity are 80 or even 100 degrees; which on a circle of 3 inches radius forms an extent sufficient for observations of this kind. M. Sauflure has even made smaller instruments that may be carried conveniently in the pocket, and to make experiments with under small receivers: they were but seven inches high by two inches of breadth; which, notwithstanding their variations, were very sensible.

Thus much for the construction of the various parts of the instrument. The limits of this work will not admit of our inferring the whole of M. Sauflure's subsequent account of the preparation of the hair, the manner of determining the limits of extreme humidity and of extreme dryness, the pyrometrical variations of the hair, and the graduation of the hygrometer. The following extract must therefore suffice.

In the preparation of the hair, it was found necessary to free it of a certain uncleanness it always has in its natural state, which in a great measure deprives it of its hygrometrical sensibility. A number of hairs are boiled in a ley of vegetable alkali; and among these are to be chosen for use such as are most transparent, bright, and soft; particular precautions are necessary for preventing the straining 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 dryness. The former is obtained by exposing the instrument to air completely saturated with water; and this is effected by placing it in a glass receiver standing in water, the sides 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 dryness, not absolute dryness, for that does not exist, exist, but the greatest degree of it that can be obtained, is produced by introducing repeatedly into the same receiver containing the instrument, and standing now upon quicksilver, certain quantities of deliquescent alkaline salts, which absorb the moisture of the air. The highest point to which the hand can be brought by this operation, not only when it will rise no higher, but when it becomes retrograde from the dilatation occasioned by heat, is called o; and the arch between these two points is divided into 100 equal parts, being degrees of the hygrometer. The arch pp, upon which the scale is marked in the instrument (represented in fig. 2.) being part of a circle of three inches diameter; hence every degree measures about one-third of a line.

In the stationary hygrometer, fig. 1. the scale upon the complete circular dial is so much larger, that every degree measures about five lines; but this M. Sauflure confesses far from being a perfection, that it is rather an inconvenience; since the instrument becomes thereby so very susceptible of the least impression, that there is even no approaching it without a sensible variation. The thermometer, adapted as before mentioned, serves to correct the changes of temperature: towards the extreme of dryness, 1° of the thermometer produces on the hair an effect of half a degree of the hygrometer, but towards the extreme of moisture, the same difference of temperature causes an effect no less than 3° on the hygrometer. He constructed two tables, that gave the intermediate hygrometrical variations for single degrees of the thermometer at different parts of the scale.

The whole range of the atmospheric variations takes in about 75° of this scale; a dryness of more than 25° being always the effect of art. The sensibility of this instrument is so very great, that being exposed to the dew, he mentions that it varies above 40° in about 20 minutes of time. Being removed from a very moist into a very dry air, it varied in one instance no less than 35° in three minutes. He says that its variations were always found uniform in different instruments suspended in different parts of the same atmosphere. This hygrometer is considered by the author as possessed of all the properties requisite in such an instrument. There are, 1. That the degrees in the scale be sufficiently large, and to point out even the least variation in the dryness or moisture of the atmosphere. 2. That it be quick in its indications. 3. That it be at all times consistent with itself; viz. that in the same state of the hair it always points to the same degree. 4. That several of them agree with one another. 5. That it be affected only by the aqueous vapours. 6. That its variations be ever proportionate to the changes in the air.

But after all it must be observed, that a considerable degree of trouble and delicacy is requisite in the preparation of the hair, and it is very fragile; circumstances which may prevent it from coming into general use among common observers, although probably it may be the best in principle of any yet made.

Instead of hairs or cat-gut, of which hygrometers of the first kind are commonly made, Caffebois, a Benedictine monk at Mentz, proposed to make such hygrometers of the gut of a silk-worm. When that insect is ready to spin, there are found in it two vessels proceeding from the head to the stomach, to which they adhere, and then bend towards the back, where they form a great many folds. The part of these vessels next the stomach is of a cylindric form, and about a line in diameter. These vessels contain a gummy sort of matter from which the worm spins its silk; and, though they are exceedingly tender, means have been devised to extract them from the insect, and to prepare them for the above purpose. When the worm is about to spin, it is thrown into vinegar, and suffered to remain there twenty-four hours; during which time the vinegar is absorbed into the body of the insect, and coagulates its juices. The worm being then opened, both the vessels, which have now acquired strength, are extracted; and, on account of their pliability, are capable of considerable extension. That they may not, however, become too weak, they are stretched only to the length of about fifteen or twenty inches. It is obvious that they must be kept sufficiently extended till they are completely dry. Before they attain to that state, they must be freed, by means of the nail of the finger, from a flinty substance which adheres to them. Such a thread will sustain a weight of six pounds without breaking, and may be used for an hygrometer in the same manner as cat-gut; but we confess that we do not clearly perceive its superiority.

II. On the second general principle, namely, that of De Luc's, the swelling of solid bodies by moisture, and their contraction by dryness, M. de Luc's instrument is the best. He makes choice of ivory for the construction of his hygrometer, because he finds that, being once wetted, ivory regularly swells by moisture, and returns exactly to the same dimensions when the moisture is evaporated, which other bodies do not. This hygrometer is represented in fig. 9. where ab is an ivory tube open Fig. 9r at the end ac, and close at b. It is made of a piece of ivory taken at the distance of some inches from the top of a pretty large elephant's tooth, and likewise at the same distance from its surface and from the canal which reaches to that point. (This particular direction is given, that the texture of the ivory in all different hygrometers may be the same, which is of great importance). This piece is to be bored exactly in the direction of its fibres; the hole must be very straight, its dimensions 2½ lines in diameter, and 2 inches 8 lines in depth from ac to c. Its bore is then to be exactly filled with a brass cylinder, which, however, must project somewhat beyond the ivory tube; and thus it is to be turned on a proper machine, till the thickness of the ivory is exactly ¼ of a line, except at the two extremities. At the bottom b the tube ends in a point; and at the top ac it must for about two lines be left a little thicker, to enable it to bear the pressure of another piece put upon it. Thus the thin or hygrometrical part of the tube will be reduced to 2½ French inches, including the concavity of the bottom. Before this piece is used, it must be put into water, so that the external part alone may be wetted by it; and here it is to remain till the water penetrates to the inside, and appears in the form of dew, which will happen in a few hours. The reason of this is, that the ivory tube remains somewhat larger ever after it is wetted the first time.

For this hygrometer, a glass tube must be provided about 14 inches long, the lower end of which is shown in ddee. Its internal diameter is about ¼ of a line. If now the ivory tube is exactly filled with mercury, and the glass be affixed to it, as the capacity of the former decreases by being dried, the mercury will be forced up into the glass one.

The piece $f'g'g$ is intended to join the ivory with the glass tube. It is of brass, shaped as in the figure. A cylindrical hole is bored through it, which holds the glass tube as tight as possible without danger of breaking it; and its lower part is to enter with some degree of difficulty into the ivory pipe. To hinder that part of the tube which incloses the brass piece from being affected by the variations of the moisture, it is covered with a brass retort represented in $h'k'ti$. The pieces must be united together with gum-lac or mastic.

The introduction of the mercury is the next operation. For this purpose, a slip of paper three inches wide is first to be rolled over the glass tube, and tied fast to the extremity nearest the ivory pipe. A horse-hair is then to be introduced into the tube, long enough to enter the ivory pipe by an inch, and to reach three or four inches beyond the extremity of the glass one. The paper which has been shaped round the tube must now be raised, and used as a funnel to pour the mercury into the instrument, which is held upright. The purest quicksilver is to be used for this purpose, and it will therefore be proper to use that revived from cinnabar. It easily runs into the tube; and the air escapes by means of the horse-hair, assisted with some gentle shakes. Fresh mercury must from time to time be supplied, to prevent the mercurial tube from being totally emptied; in which case, the mercurial pellicle which always forms by the contact of the air, would run in along with it.

Some air-bubbles generally remain in the tube; they may be seen through the ivory pipe, which is thin enough to have some transparency. These being collected together by shaking, must be brought to the top of the tube, and expelled by means of the horse-hair. To facilitate this operation, some part of the mercury must be taken out of the tube, in order that the air may be less obstructed in getting out, and the horse-hair have a free motion to assist it. Air, however, cannot be entirely driven out in this manner. It is the weight of the mercury with which the tube is for that reason to be filled, which in time completes its expulsion, by making it pass through the pores of the ivory. To hasten this, the hygrometers are put into a proper box. This is fixed nearly in a vertical direction to the saddle of a horse, which is set a trotting for a few hours. The shakes sometimes divide the column of mercury in the glass tube, but it is easily re-united with the horse-hair. When upon shaking the hygrometer vertically, no small tremulous motion is any longer perceived in the upper part of the column, one may be sure that all the air is gone out.

The scale of this hygrometer may be adjusted, as soon as the air is gone out, in the following manner. The instrument is to be suspended in a vessel of water cooled with ice, fresh quantities of which are to be added as the former melts. Here it is to remain till it has sunk as low as it will sink by the enlargement of the capacity of the ivory tube, owing to the moisture it has imbibed. This usually happens in seven or eight hours, and is to be carefully noted. In two or three hours the mercury begins to ascend, because the moisture passes into the cavity, and forces it up. The lowest station of the mercury is then to be marked; and for the more accurate marking the degrees on the scale, M. de Luc always chose to have his hygrometric tube made of one which had formerly belonged to a thermometer. The reason of this is, that in the thermometer the expansion of the mercury by heat had been already determined. The distance between the thermometrical points of melting ice and boiling water at 27 French inches of the barometer was found to be 1937 parts. The bulb of this preparatory thermometer was broke in a basin, in order to receive carefully all the mercury that it contained. This being weighed in nice scales amounted to 1428 grains. The hygrometer contained 460 grains of the same mercury. Now it is plain, that the extent of the degrees on the hygrometer, ought to be to that of the degrees on the preparatory thermometer as the different weights of the mercury contained in each; consequently 1428 : 460 : 1937 : 624 nearly; and therefore the corresponding intervals ought to follow the same proportion: and thus the length of a scale was obtained, which might be divided into as many parts as he pleased.

Fig. 10. is a representation of De Luc's hygrometer when fully constructed. In elegance it far exceeds Smeaton's or any other, and probably also in accuracy; for by means of a small thermometer fixed on the board along with it, the expansion of the mercury by heat may be known with great accuracy, and of consequence how much of the height of the mercury in the hygrometer is owing to that cause, and how much to the mere moisture of the atmosphere.

M. de Luc having continued his inquiries further into the modifications of the atmosphere, mentions in his Idée sur la Météorologie another hygrometer, which he finds to be the best adapted to the measure of local humidity. Of all the hygroscopic substances which he tried for this purpose, that which answers the best is a slip of whalebone cut transversely to the direction of the fibres, and made extremely thin; for on this depends its sensibility. A slip of 12 inches in length and a line in breadth, he has made so thin as to weigh only half a grain; and it may be made still thinner, but is then of too great sensibility, being affected even by the approach of the observer. This slip is kept extended by a small spring, and the variations in its length are measured by a vernier division, or by, which is perhaps better, an index on a dial plate: the whole variation from extreme dryness to extreme moisture is about $\frac{1}{8}$ of its length.

In these hygrometers, which are made by the instrument-makers in London, the slip of whalebone is mounted in a frame very similar to that belonging to M. Saussure's hygrometer before described (see fig. 7.). The only material difference is, that a small concentric wire spring is used, instead of a counterpoise, to keep the slip of whalebone extended. M. Saussure had tried such a spring applied to his hairs; but the weakest spring he found too strong for the hair; and he was further apprehensive, that the variations which the cold, heat, and the weather infallibly make, would suffer from the force of the springs.

M. de Luc, in the hygrometers he formerly made, as before described (made of ivory), had graduated them from one fixed point only, that of extreme moisture, which is obtained by soaking them in water. He has now very ingeniously contrived to fix the other extreme, that of dryness; but this being producible only by means of strong fires, such as hygrometers cannot support, he uses an intermediate body, quicklime; which after having been deprived, by force of fire, of all its own humidity, has the property of slowly imbibing humidity again from the bodies in its neighbourhood; and whose capacity is such that all the vapour that can be contained in a quantity of air equal to its own bulk, can give it no sensible humidity. These hygrometers, included with a large quantity of fresh burnt lime in lumps, acquire in three weeks the same degree of dryness with the lime, which cannot differ sensibly from extreme dryness.

M. de Sauffure makes choice of hairs, prepared by maceration in alkaline lye. M. de Luc shows that hairs, and all other animal or vegetable substances, taken lengthwise, or in the direction of their fibres, undergo contrary changes from different variations of humidity; that, when immersed in water, they lengthen at first, and afterwards shorten; that when they are near the greatest degree of humidity, if the moisture is increased, they shorten themselves; if it is diminished, they lengthen themselves first before they contract again. These irregularities, which obviously render them incapable of being true measures of humidity, he shows to be the necessary consequence of their organic reticular structure.

M. de Sauffure takes his point of extreme moisture from the vapours of water under a glass bell, keeping the sides of the bell continually moistened; and affirms, that the humidity is there constantly the same in all temperatures; the vapours even of boiling water having no more effect than those of cold. M. de Luc shows, on the contrary, that the differences of humidity under the bell are very great, though M. Sauffure's hygrometer was incapable of discovering them; and that the real undecomposed vapour of boiling water has the directly opposite effect to that of cold, the effect of extreme dryness; and on this point he mentions an interesting fact, communicated to him by Mr Watt, viz. that wood cannot be employed in the steam engine for any of those parts where the vapour of the boiling water is confined, because it dries so as to crack, just as it exposed to the fire. In M. de Luc's work above mentioned there are striking instances related, in which the imperfection of M. Sauffure's hygrometer led him into false conclusions respecting phenomena, and into erroneous theories to account for them.

III. On the third principle, namely, the alteration of the weight of certain substances by their attracting the moisture of the air, few attempts have been made, nor do they seem to have been attended with much success. Sponges dipped in a solution of alkaline salts, and some kinds of paper, have been tried. These are suspended to one end of a very accurate balance, and counterpoised by weights at the other, and show the degrees of moisture or dryness by the ascent or descent of one of the ends. But, besides that such kinds of hygrometers are destitute of any fixed point from whence to begin their scale, they have another inconvenience (from which indeed Smeaton's is not free, and which has been found to render it erroneous), namely, that all saline substances are destroyed by long continued exposure to the air in very small quantities, and therefore can only imbibe the moisture for a certain time. Sulphuric acid has therefore been recommended in preference to the alkaline or neutral salts, and, indeed, for such as do not choose to be at the trouble of constructing a hygrometer on the principles of Mr Smeaton or De Luc, this will probably be found the most easy and accurate.

Fig. II. represents an hygro-meter of this kind. A is a small glass cup containing a small quantity of oil of vitriol, B an index counterpoising it, and C the scale; where it is plain, that as the oil of vitriol attracts the moisture of the air, the scale will descend, which will raise the index, and vice versa. This liquor is exceedingly sensible of the increase or decrease of moisture. A single grain, after its full increase, has varied its equilibrium so sensibly, that the tongue of a balance, only an inch and a half long, has described an arch one-third of an inch in compass (which arch would have been almost three inches if the tongue had been one foot), even with so small a quantity of liquor; consequently, if more liquor, expanded under a large surface, were used, a pair of scales might afford as nice an hygrometer as any kind yet invented. A great inconvenience, however, is, that as the air must have full access to the liquid, it is impossible to keep out the dust, which, by continually adding its weight, must render the hygrometer false; add to this, that even oil of vitriol itself is by time destroyed, and changes its nature, if a small quantity of it is continually exposed to the air.

The best hygrometer upon this principle, and for ascertaining the quantity as well as the degree of moisture in the variation of the hygrometer, is of the contrivance of Mr Coventry, Southwark, London. The account he has favoured us with is as follows.

"Take two sheets of fine tissue paper, such 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 scales no moisture remains. When the sheets are in this perfectly dry state, reduce them to exactly 50 grains; the hygrometer is then fit for use. The sheets must be kept free from dust, and exposed a few minutes in the open air; after which it may be always known by weighing them the exact quantity of moisture they have imbibed.

"For many years the hygrometer has (says Mr Coventry) engrossed a considerable share of my attention; and every advantage proposed by others, either as it respected the substances of which the instrument was composed, or the manner in which its operations were to be discerned, has been impartially examined. But (adds he) I have never seen an hygrometer so simple in itself, or that would act with such certainty or so equally alike, as the one I have now described. The materials of which it is composed being thin, are easily deprived wholly of their moisture; which is a circumstance essentially necessary in fixing a datum from which to reckon, and which, I think, cannot be said of any substance hitherto employed in the construction of hygrometers; with equal facility they imbibe or impart the humidity of the atmosphere, and show with the greatest exactness when the least alteration takes place." When the paper is prepared, as already described, it will serve, without the trouble of drying, as a standard for any number of sheets intended for the same purpose. But then the sheets must be kept together in the open air for a few hours; because whatever alteration may take place by this exposure, the paper already weighed must have undergone the same; being consequently in the same state, they must be cut to the same weight.

For easier weighing the paper, take a piece of round tin or brass the size of a crown piece, through the centre of which drill a hole, and also three others round it at equal distances; then cut about one hundred papers; and after putting them under the tin or brass, drive through each hole a strong pin into a board, in order to round them to the shape of the plate: the papers must be then 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, threadle them together through those holes made by the pins, putting between every paper on each thread a small bead, in order to prevent the papers from touching each other, and also that the air may be more readily admitted. The top of the hygrometer is covered with a card cut to the same size; and which, by reason of its stiffness, supports all the papers, and keeps them in proper shape. Before the papers are threaded, the beads, silk, card, and a thin piece of brass about the size of a sixpence, which must be placed at the bottom, and through which the centre string passes, must be weighed with the greatest exactness, in order to bring them to a certain weight, suppose 50 grains; now the paper in its driest state being of equal weight, they will weigh together 100 grains, consequently what they weigh more at any time is moisture.

To obviate the trouble and difficulty of making experiments with weights and scales, Mr Coventry contrived a machine or scale by which to determine at one view the humidity or dryness of the atmosphere. This, with its case, is represented by fig. 12. The front and back of the case are glass; the sides fine gauze, which excludes the dust and admits the air; the case is about ten inches high, 8 inches broad, and 4 inches deep. A, a brass bracket in front, behind which, at about 3½ inches distance is another; these support the axis of the index E, also of the beam D, and another which supports the stem B, to which the ivory scale of divisions C is fixed. G, a brass scale suspended in the usual manner to the end of a beam D, and weighing exactly 100 grains. This scale is an exact counterpoise to the papers I and the different apparatus. The particular manner of suspension in this balance is, from the construction, 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 steel edges k k, fixed in the extremities of the brass axis: these edges are shaped like the edge of a knife, and act on two steel concave edges l l, in order to render the friction as small as possible. D, is a fine scale beam, fixed at right angles with the axis g. E, the steel index fixed to the under side of the same axis. F, a brass sliding weight: h is the axis that holds the stem B to which the scale of divisions C is fixed. AA, the brass brackets which support the whole by four screws, two of which are seen at i i, that screw the hygrometer brackets to the top of the case. The axis of the scale of divisions is hung on pivots, one of which is seen at m, that should the case not stand level, the stem B may always be in a perpendicular situation.

The hygrometer, before use, should be adjusted as follows: To the end of the beam where the hygrometer is suspended, hang a weight of 100 grains, which is equal to the weight of the scale; then move the sliding weight F up or down the index E, till one grain will cause the index to traverse neither more nor less than the whole scale of divisions; then add half a grain to the scale, in order to bring the index to 0; and the instrument, after taking off the 100 grain weight and hanging on the papers, is fit for use; then put grain weights in the scale till the index is brought within compass of the scale of divisions. Example: H is 3 grains on the brass scale, and the index points at 10; consequently there is 3 grains and 10 hundredths 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 traversing within the scale of divisions for many days without shifting the weights; but if otherwise, they must be changed as occasion may require.

"One great advantage of this hygrometer above all others that have attracted my notice is (says Mr Coventry), that it acts from a certain datum, namely, the dry extreme; from which all the variations towards moist are calculated with certainty; and if constructed with that precision represented by the drawing, it will afford pleasure to the curious in observing the almost perpetual alteration of the atmosphere, even in the most settled weather. In winter it will be constantly traversing from about eight in the morning till four or five in the afternoon, towards dry; and in summer, from about four in the morning till six or seven in the evening, when the weather is hot and gloomy, the hygrometer discovers a very great change towards moisture; and when clear and frosty, that it contains a much greater quantity of moisture than is generally imagined."

An improvement has been proposed of this kind of hygrometer, of which the following circumstance, it is said, suggested the first hint. While Mr Lowitz was at Dmitrievsk in Astracan, he found, on the banks of the Wolga, a thin bluish kind of flake which attracted moisture remarkably soon, but again suffered it as soon as it escaped. A plate of this flake weighed, when brought to a red heat, 175 grains, and, when saturated with water, 247; it had therefore imbibed, between complete dryness and the point of complete moisture, 72 grains of water. Lowitz suspended a round thin plate of this flake at the end of a very delicate balance, fastened within a wooden frame, and suspended at the other arm a chain of silver wire, the end of which was made fast to a sliding nut that moved up and down in a small groove on the edge of one side of the frame. He determined, by trial, the position of the nut when the balance was in equilibrium and when it had ten degrees of over-weight, and divided the space between these two points into ten equal parts, adding such a number more of these parts as might be necessary. When the stone was suspended from the one arm HYL

Hygrometer of the balance, and at the other a weight equal to 175 grains, or the weight of the stone when perfectly dry, the nut in the groove shewed the excess of weight in grains when it and the chain were so adjusted that the balance stood in equilibrio. A particular apparatus on the same principles as a vernier, applied to the nut, shewed the excess of weight to ten parts of a grain.

Lowitz remarked that this hygrometer in continued wet weather gave a moisture of more than 15 grains, and in a continued heat of 113 degrees of Fahrenheit only 1½ degree of moisture.

The hygrometer thus invented by Lowitz was, however, attended with this fault, that it never threw off the moisture in the same degree as the atmosphere became drier. It was also sometimes very deceitful, and announced moisture when it ought to have indicated that dryness had again begun to take place in the atmosphere. To avoid these inconveniences, M. Hochheimer proposes the following method:

1. Take a square bar of steel about two lines in thickness, and from ten to twelve inches in length, and form it into a kind of balance, one arm of which ends in a screw. On this screw let there be forewed a leaden bullet of a proper weight, instead of the common weights that are suspended.

2. Take a glass plate about ten inches long, and seven inches in breadth; destroy its polish on both sides, free it from all moisture by rubbing it over with warm ashes, suspend it at the other end of the balance, and bring the balance into equilibrium by screwing up or down the leaden bullet.

3. Mark now the place to which the leaden bullet is brought by the screw, as accurately as possible, for the point of the greatest dryness.

4. Then take away the glass plate from the balance, dip it completely in water, give it a shake that the drops may run off from it, and wipe them carefully from the edge.

5. Apply the glass plate thus moistened again to the balance, and bring the latter into equilibrium by screwing the leaden bullet. Mark then the place at which the bullet stands as the highest degree of moisture.

6. This apparatus is to be suspended in a small box of well dried wood, sufficiently large to suffer the glass plate to move up and down. An opening must be made in the lid, exactly of such a size as to allow the tongue of the balance to move freely. Parallel to the tongue apply a graduated circle, divided into a number of degrees at pleasure, from the highest point of dryness to the highest degree of moisture. The box must be pierced with small holes on all the four sides, to give a free passage to the air; and to prevent moisture from penetrating into the wood by rain, when it may be requisite to expose it at a window, it must either be lacquered or painted. To save it at all times from rain, it may be furnished with a sort of roof.

For a description of Mr Leclerc's Hygrometer, fig. 13, and in a more portable form, fig. 14. See Meteorology Index.