HYGROMETER

, or Hygroscope, or NOTIOMETER, an instrument for measuring the degrees of <*>oisture in the air.

There are various kinds of Hygrometers; for whatever body either swells by moisture, or shrinks by dryness, is capable of being formed into an Hygrometer. Such are woods of most kinds, particularly deal, ash, poplar, &c. Such also is catgut, the beard of a wild oat, and twisted cord, &c. The best and most usual contrivances for this purpose are as follow.

1. Stretch a common cord, or a fiddle-string, ABD along a wall, passing it over a pulley B; fixing it at one end A, and to the other end hanging a weight E, carrying a style or index F. Against the same wall fit a plate of metal HI, graduated, or divided into any number of equal parts; and the Hygrometer is complete.

For it is matter of constant observation, that moisture sensibly shortens cords and strings; and that, as the moisture evaporates, they return to their former length again. The like may be said of a fiddle-string: and from hence it happens that such strings are apt to break in damp weather, if they are not slackened by the screws of the violin. Hence it follows, that the weight E will ascend when the air is more moist, and descend again when it becomes drier. By which means the index F will be carried up and down, and, by pointing to the several divisions on the scale, will shew the degrees of moisture or dryness.

2. Or thus, for a more sensible and accurate Hygro- meter: strain a whipcord, or catgut, over several pulleys B, C, D, E, F; and proceed as before for the rest of the construction. Nor does it matter whether the several parts of the cord be parallel to the horizon, as expressed in the annexed figure, or perpendicular to the same, or in any other position; the advantage of this, over the former method, being merely the having a greater length of cord in the same compass; for the longer the cord, the greater is the contraction and dilatation, and consequently the degrees of variation of the index over the scale, for any given change of moisture in the air.|

3. Or thus: Fasten a twisted cord, or fiddle-string, AB, by one end at A, sustaining a weight at B, carrying an index C round a circular scale DE described on a horizontal board or table.—For a cord or catgut twists itself as it moistens, and untwists again as it dries. Hence, upon an increase or decrease of the humidity of the air, the index will shew the quantity of twisting or untwisting, and consequently the increase or decrease of moisture or dryness.

4. Those Dutch toys, called weather houses, where a small image of a man, and one of a woman, are fixed upon the ends of an index, are constructed upon this principle. For the index, being sustained by a cord or twisted catgut, turns backwards and forwards, bringing out the man in wet weather, and the woman in dry.

5. Or thus: Fasten one end of a cord, or catgut, AB, to a hook at A; and to the other end a ball D of about one pound weight; upon which draw two concentric circles, and divide them into any number of equal parts, for a scale; then fit a style or index EC into a proper support at E, so as the extremity C may almost touch the divisions of the ball.——Here the cord twisting or untwisting, as in the former case, will indicate the change of moisture, by the successive application of the divisions of the circular scale, as the ball turns round, to the index C.

6. Or an Hygrometer may be made of the thin boards of ash or <*>ir, by their swelling or contracting. But this, and all the other kinds of this instrument, above described, become in time sensibly less and less accurate; till at last they lose their effect entirely, and suffer no alteration from the weather. But the following sort is much more durable, serving for many years with tolerable accuracy.

7. Take the Manoscope, described under that article, and instead of the exhausted ball E, substitute a sponge, or other body, that easily imbibes moisture. To prepare the sponge, it may be proper first to wash it in water very clean; and, when dry again, in water or vinegar in which there has been dissolved sal ammoniac, or salt of tartar; after which let it dry again.——Now, if the air become moist, the sponge will imbibe it and grow heavier, and consequently will preponderate, and turn the index towards C; on the contrary, when the air becomes drier, the sponge becomes lighter, and the index turns towards A; and thus shewing the state of the air.

8. In the last mentioned Hygrometer, Mr. Gould, in the Philos. Trans. instead of a sponge, recommends oil of vitriol, which grows sensibly lighter or heavier from the degrees of moisture in the air; so that being saturated in the moistest weather, it afterwards retains or loses its acquired weight, as the air proves more or less moist. The alteration in this liquor is so great, that in the space of 57 days it has been known to change its weight from 3 drachms to 9; and has shifted a tongue or index of a balance 30 degrees. So that in this way a pair of scales may afford a very nice Hygrometer. The same author suggests, that oil of sulphur or campanam, or oil of tartar per deliquium, or the liquor of fixed nitre, might be used instead of the oil of vitriol.

9. This balance may be contrived in two ways; by either having the pin in the middle of the beam, with a slender tongue a foot and a half long, pointing to the divisions on an arched plate, as represented in the last figure above. Or the scale with the liquor may be hung to the point of the beam near the pin, and the other extremity made so long, as to describe a large arch on a board placed for the purpose; as in the figure here annexed.

10. Mr. Arderon has proposed some improvement in the sponge Hygrometer. He directs the sponge A to be so cut, as to contain as large a superficies as possible, and to hang by a fine thread of silk upon the beam of a| balance B, and exactly balanced on the other side by another thread of silk at D, strung with the smallest lead shot, at equal distances, so adjusted as to cause an index E to point at G, the middle of a graduated arch FGH, when the air is in a middle state between the greatest moisture and the greatest dryness. Under this silk so strung with shot, is placed a little table or shelf I, for that part of the silk or shot to rest upon which is not suspended. When the moisture imbibed by the sponge increases its weight, it will raise the index, with part of the shot, from the table, and vice versa when the air is dry. Philos. Trans. vol. 44, p. 96.

11. From a series of Hygroscopical observations, made with an apparatus of deal wood, described in the Philos. Trans. number 480, Mr. Coniers concludes, 1st, That the wood shrinks most in summer, and swells most in winter, but is most liable to change in the spring and fall. 2d, That this motion happens chiefly in the day time, there being scarce any variation in the night. 3d, That there is a motion even in dry weather, the wood swelling in the morning, and shrinking in the afternoon. 4th, That the wood, by night as well as by day, usually shrinks when the wind is in the north, north-east, and east, both in summer and winter. 5th, That by constant observation of the motion and rest of the wood, with the help of a thermometer, the direction of the wind may be told nearly without a weather-cock. He adds, that even the time of the year may be known by it; for in spring it moves more and quicker than in winter; in summer it is more shrunk than in spring; and has less motion in autumn than in summer.

See an account of a method of constructing these and other Hygrometers, in Phil. Trans. Abr. vol. 2, p. 30, &c, and plate 1 annexed. See also Philos. Trans. vol. 11. p. 647 and 715, vol. 15, p. 1032, vol. 43, p. 6, vol. 44, p. 95, 169 and 184, vol. 54, p. 259, vol. 61, p. 198, vol. 63, p. 404, &c.

12. Dr. Hook's Hygrometer was made of the beard of a wild oat, set in a small box, with a dial plate and an index. See his Micrographia, p. 150.

13. The Doctors Hales and Desaguliers both contrived another form of sponge Hygrometer, on this principle. They made an horizontal axis, having a small part of its length cylindrical, and the remainder tapering conically with a spiral thread cut in it, after the manner of the fuzee of a watch. The sponge is suspended by a fine silk thread to the cylindrical part of the axis, upon which it winds. This is balanced by a small weight W, suspended also by a thread, which winds upon the spiral fuzee. Then when the sponge grows heavier, in moist weather; it descends and turns the axis, and so draws up the weight, which coming to a thicker part of the axis it becomes a balance to the sponge, and its motion is shewn by an attached scale. And vice versa when the air becomes drier.——Salt of tartar, or any other salt, or pot ashes, may be put into the scale of a balance, and used instead of the sponge. Desag. Exper. Philos. vol. 2, p. 300.

14. Mr. Ferguson made an Hygrometer of a thin deal pannel; and to enlarge the scale, and so render its variations more sensible, he employed a wheel and axle, making one cord pass over the axle, which turned a wheel ten times as large, over which passed a line with a weight at the end of it, whose motion was therefore ten times as much as that of the deal pannel. The board should be changed in 3 or 4 years. See Philos. Trans. vol. 54, art. 47.

15. Mr. Smeaton gave also an ingenious and elaborate construction of an Hygrometer; which may be seen in the Philos. Trans. vol. 61, art. 24.

16. Mr. De Luc's contrivance for an Hygrometer is very ingenious, and on this principle. Finding that even ivory swells with moisture, and contracts with dryness, he made a small and very thin hollow cylinder of ivory, open only at the upper end, into which is fitted the under or open end of a very fine long glass tube, like that of a thermometer. Into these is introduced some quicksilver, silling the ivory cylinder, and a small part of the length up the glass tube. The consequence is this: when moisture swells the ivory cylinder, its bore or capacity grows larger, and consequently the mercury sinks in the fine glass tube; and vice versa, when the air is drier, the ivory contracts, and forces the mercury higher up the tube of glass. It is evident that an instrument thus constructed is in fact also a thermometer, and must necessarily be affected by the vicissitudes of heat and cold, as well as by those of dryness and moisture; or that it must act as a thermometer as well as an Hygrometer. The ingenious contrivances in the structure and mounting of this instrument may be seen in the Philos. Trans. vol. 63, art. 38; where it may be seen how the above imperfection is corrected by some simple and ingenious expedients, employed in the original construction and subsequent use of the instrument; in consequence of which, the variations in the temperature of the air, though they produce their full effects on the instrument, as a thermometer, do not interfere with or embarrass its indications as an Hygrometer.

17. In the Philos. Trans. for 1791, Mr. De Luc has given a second paper on Hygrometry. This has been chiefly occasioned by a Memoir of M. de Saussure on the same subject, entitled Essais sur l'Hygrometrie, in 4to, 1783. In this work M. de S. describes a new Hygrometer of his construction, on the following principle. It is a known fact that a hair will stretch when it is moistened, and contract when dried: and M. de Saussure found, by repeated experiments, that the difference between the greatest extension and contraction, when the hair is properly prepared, and has a weight of about 3 grains suspended by it, is nearly one 40th of its whole length, or one inch in 40. This circumstance suggested the idea of a new Hygrometer. To| render these small variations of the length of the hair perceptible, an apparatus was contrived, in which one of the extremities of the hair is fixed, and the other, bearing the counterpoise abovementioned, surrounds the circumference of a cylinder, which turns upon an axis to which a hand is adapted, marking upon a dial in large divisions the almost insensible motion of this axis. About 12 inches high is recommended as the most convenient and useful: and to render them portable, a contrivance is added, by which the hand and the counterpoise can be occasionally fixed.

But M. de Luc, in his Idées sur la Meteorologie, vol. 1, anno 1786, shews that hairs, and all the other animal or vegetable hygroscopic 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 be increased, they shorten themselves; if it be diminished, they lengthen themselves first before they contract again. These irregularities, which render them incapable of being true measures of humidity, he shews to be the necessary consequence of their organic reticular structure De Saussure 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 other effect than those of cold. De Luc, on the contrary, shews that the differences in humidity under the bell are very great, though De Saussure's Hygrometer was not capable 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 as if exposed to the fire.

To these charges of M. De Luc, a reply is made by M. De Saussure, in his Defence of the Hair Hygrometer, in 1788; where he attributes the general disagreement between the two instruments, to irregularities of M. De Luc's; and assigns some aberrations of his own Hygrometer, which could not have proceeded from the above cause, but to its having been out of order; &c.

This has drawn from M. De Luc a second paper on Hygrometry, published in the Philos. Trans. for 1791, p. 1, and 389. This author here resumes the four fundamental principles which he had sketched out in the former paper, viz, 1st, That fire is a sure, and the only sure means of obtaining extreme dryness. 2d, That water, in its liquid state, is a sure, and the only sure means of determining the point of extreme moisture. 3d, There is no reason, a priori, to expect, from any hygroscopic substance, that the measurable effects, produced in it by moisture, are proportional to the intensities of that cause.—But, 4th, perhaps the comparative changes of the di<*>ensions of a substance, and of the weight of the same or other substances, by the same variations of moisture, may lead to some discovery in that respect. On these heads M. De Luc expatiates at large in this paper, shewing the imperfections of M. De Saussure's principles of Hygrometry, and particularly as to a hair, or any such substance when extended lengthwise, being properly used as an Hygrometer. On the other hand, he shews that the expansion of substances across the fibres, or grain, renders them, in that respect, by far the most proper for this purpose. He chooses such as can be made very thin, as ivory, or deal shavings, but over all he finds whalebone to be far the best of any. But, for all the reasonings of these ingenious philosophers on this interesting subject, and complete information, see the publications above quoted, as also the Monthly Review, vol. 51, p. 224, vol. 71, p. 213, vol. 76, p. 316, vol. 78, p. 236, and vol. 6, of the new series for the year 1791, p. 133.