HYGROMETRY. In the British Islands all are familiar with the arid character of the east winds of spring, and not a few are only too painfully aware of the discomfort experienced while under their influence ; and all are like-wise familiar with the opposite state of the atmosphere, most frequently and unmistakably occurring also with east winds, when every object feels damp and clammy to the touch, and horses on the streets are seen each with a steam-ing cloud of dense mist around it. In certain other climates, such as are met with in India and South Africa, these effects are greatly intensified, so that on the one hand the ivory scales of thermometers, quill pens, and other objects curl up, articles of furniture open at the joints and split up, and the grass which covers the soil is reduced to a state of tinder ; and on the other hand, everything becomes so permeated with moisture that, even in the interior of houses, furniture, books, and wearing apparel become sodden with wet. These different effects depend on the states of the air as regards the quantity of aqueous vapour diffused through it taken in connexion with the temperature, these varying from the completest possible saturation of the air, which is of occasional occurrence in the rainy season of some tropical climates, to that extreme desiccation of the air which sometimes happens in Great Britain in spring, but more completely and frequently in such dry summer climates as that of the Punjaub.
A large number of substances, such as sugar, flour, and bread, possess the property of absorbing moisture, and most gases, as well as air, absorb and retain aqueous vapour. The term hygrometry is employed to signify the measure-ment of the degree of dampness of substances, and to denote the processes by which their humidity is ascertained. The term, however, may be considered as restricted to the humidity of the atmosphere, owing to the paramount im-portance of that branch of the subject, and the slight and unsatisfactory knowledge we yet possess of the laws of hygrometry of other substances.
All organic substances contain pores for the conveyance of their juices, and are influenced by the accession of mois-ture, some of them very markedly so. Every species of wood is liable to these hygrométrie changes, the amount of contraction and expansion being much greater across the grain of the wood than lengthways. Hence the panels of doors are fitted into grooves so as to allow of shrinkage, for, if secured at the edges, the panels must inevitably split. The hair of animals is also eminently hygrométrie, curling and uncurling as the air becomes drier or moister, and it is because of the peculiar sensations accompanying these hygrométrie changes that the cries and behaviour of many of the lower animals furnish valuable prognostications of weather changes. Similarly many manufactured objects, such as paper, cordage, &c, vary in weight, bulk, form, and elasticity with the varying degrees of humidity of the air, and other interesting prognostics have been drawn from these hygrométrie changes.
In the earlier stages of the investigation of the hygro-metry of the air, the hygrométrie properties of several sub-stances were made use of as instruments of observation. Of these may be named the twisted Indian grass (OobeenaIIooloo), employed by Captain Kater on account of its remarkable property of twisting and untwisting according to the dampness of the air; a slip of whalebone cut across the fibres, used by De Luc; and the hygrometer of Saussure, which was formed of a hair from which the oily matter had been previously removed, and which stretches when moist and contracts when dry. Experience has, however, shown that none of these hygrometers are satisfactory instruments, seeing they give inconsistent results, and are liable when in use to great and uncertain changes. Hence, while they serve to give the roughest idea of the state of the air as regards moisture, they have fallen into disuse as accurate instruments of observation of the hygrometry of the atmo-sphere. But in the intensely cold climates of Russia and Siberia, the hair hygrometer still continues to be used as an instrument of observation; and when we consider the tediousness and difficulty of making hygrometric observa-tions with the hygrometer in most general use when the temperature of the air descends below the freezing-point of water, the hair hygrometer should perhaps be considered as good an instrument as is available to put into the hands of ordinary observers in times of low temperature.
The most accurate hygrometers are those which are con-structed, not as the above, on the principle of absorption of vapour, but on the principles of condensation or evaporation. The well-known fact that the tempera-ture of a wet body is lower than that of a dry one when under the same atmospheric conditions was applied by Sir John Leslie to mea-sure the humidity of the air. Leslie's hygrometer (fig. 1), which is an adaptation of his differential thermometer, is formed by uniting two tubes having a ball blown on the end of each, into which some coloured sulphuric ether has been previously introduced. When both bulbs are at the same temperature, the fluid stands at the zero of the scale, but when one of them is covered with wetted paper or muslin the instrument shows the depression of temperature of the wetted bulb. In order to ascer-tain the quantity of moisture cor-responding to the reading of a Leslie's hygrometer, we must de-duct from the total quantity of moisture which the air of the tem-perature at the time of observa-tion is capable of holding the deficiency due to the degree of cooling shown by the hygrometer.
As no air is ever absolutely dry, but contains more or less moisture, it is evident that if any mass of air be cooled sufficiently it may be made to deposit its moisture. A familiar example of the condensation of vapour is seeu in the formation of dew on a tumbler filled with cold water and brought into a warm room. This dew is caused by the deposition of moisture from the air in contact with the cold surface of the glass, which is cooled down below the point of saturation. The temperature of the glass at the instant dew begins to form on its surface is termed the dew-point, which corresponds with the point of saturation of the air.
Daniell's and Regnault's hygrometers are constructed on the principle of this simple phenomenon, various contriv-ances being used for lowering the temperature quickly to any point that may be desired, and for observing with requisite precision the temperature at which the dew begins to form. In both cases ether is employed to lower the temperature.
Daniell's hygrometer consists of a glass tube bent at right angles at two points with a bulb at each extremity, one bulb being of black and the other of clear glass, the latter covered with muslin. The liquid within the bulbs is ether, which at the time of being sealed is made to boil for the purpose of expelling the air. If the temperature of the two bulbs be made to differ from each other, all the ether is transferred from the warmer to the colder bulb. In making an observation the whole of the ether is first trans-ferred to the black bulb, and ether is then dropiped on the muslin covering outside the clear bulb. This ether quickly evaporates, and in doing so rapidly lowers the temperature of the clear bulb so that the ether inside the black bulb distils over into the clear bulb. The result is a lowering of the temperature of the black bulb, and, as soon as this falls to the temperature of the dew-point of the air where the experiment is conducted, a ring of vapour begins to be formed outside the black bulb, more or less dulling its surface. At this instant a thermometer placed inside the tube with its bulb immersed in the ether filling the black bulb is read, and the reading gives the dew-point of the air at the time.
Regnault's hygrometer is a little more complicated than Daniell's, but its indications are much more trustworthy. It consists (fig. 2) of a glass tube or cap-sule A, having on the bottom and a little way up a highly polished silver surface, and closed by a cork with two holes. Through one of these holes the stem of a thermometer B passes, having its bulb at the bottom of the silvered capsule, while through the other hole passes a narrow metallic tube C, one end of which opens close to the bottom of the capsule, and the other end may, if desired, be connected with an aspirator or air-pump. In making an observation as much ether is introduced into the capsule A as will cover the bulb of the thermometer, and then by trans-mitting air through the tube C the ether vapour is withdrawn from the capsule A through another tube D. By this means the temperature of the ether is very rapidly reduced, and since the whole mass of the ether is agitated by the air-bubbles which rise through it from the bottom of the capsule, the cooling of the ether is equal throughout. The thermometer is then read quickly, but to ensure an accuracy to the tenth of a degree a second and a third experiment, conducted more slowly, should be made. As showing the rapidity with which observations can be made with this hygrometer, Henry F. Blanford on occasion made six observations in six minutes in the dry climate of Secundera-bad, when the temperature of the air was 93°, the dew-point 51°, and the relative humidity consequently 24°. The temperature of the air at the time of observation may be ascertained in the usual way, or by means of the thermo-meter E, if care be taken that its temperature is unaffected by the proximity of the person of the observer.
Owing to the expense and great trouble attending the use of hygrometers which give the dew-point directly by con-densation, another hygrometer has come into extensive use by which the dew-point is determined indirectly by evapo-ration. This is August's hygrometer, sometimes called
Mason's hygrometer, but more generally known as the dry and wet bulb hygrometer, which has the strong recom-mendation of being self-acting, and requiring only the read-ings of the two thermometers in making air observations. This hygrometer (fig. 3) consists of two thermometers a and b similar to each other in all re-spects except that one of them has a piece of muslin tied closely over the surface of its bulb c, and kept constantly wet by a few threads of cotton which con-nect it with the water in the vessel d. The water then which rises from the vessel by capil-lary attraction spreads over the muslin, and evaporates from its surface with more or less rapidity according to the dryness or moist-Bess of the air ; and the greater the dryness of the air the greater is the difference between the observed readings of the dry and the wet thermometers.
The formula for deducing the hygrométrie state of the air from these two observations has been investigated by Professor Apjohn (Trans. Roy. Irish Acad., vol. xvii.), and has been already de-scribed (see ATMOSPHERE, vol. iii. p. 32). As it is very troublesome to go through the calculations for each fresh observation, tables have been prepared which give the dew-point by inspection. The best of these tables in English measures are those of Glaisher, fifth edition, constructed empirically from direct experiments carried on at Green-wich, combined with Kegnault's last revised tables relat-ing to aqueous vapour. There can be but one opinion as to the great service rendered to meteorology by Glaisher in the preparation of these tables, which give results approximately correct for high and moderate humidities and for situations at no great height above the sea ; in other words, they may be regarded as accurate for at least such conditions as are presented by the climates of the British Islands. They are, however, insufficient, owing to the com-paratively large errors attending their use, for the reduction of observations made in elevated situations and in such arid states of the atmosphere as are of frequent occurrence in India and South Africa. The preparation of such tables remains still a serious desideratum in meteorology; and another desideratum equally important is the introduction of a simple, handy, and accurate method of observing the hygrometry of the air when its temperatnre descends below the freezing-point of water, some method which would in-volve only a minimum of manipulative skill and trouble in making an observation. See ATMOSPHERE. (A. B.)