A vast number of substances, such as sugar, flour, bread, &c., possess the property of absorbing moisture, the amount of which varies according to the circumstances in which they are placed. Atmospheric air also, and most gases, absorb and retain aqueous vapour, so that in all experiments regarding the composition of bodies it is necessary to ascertain their state as to dampness. The values of many commodities are greatly influenced by the quantity of moisture which they hold, and hence the utility of having some means of ascertaining this quantity.

The word hygrometry (from ὑγρός, moist, and μέτρον, a measure) signifies the measurement of dampness, and therefore serves to denote the assemblage of processes, by help of which we attempt to measure the humidity of bodies. But the word is commonly restricted to atmospheric hygrometry; and, indeed, is the only department which has received adequate attention from natural philosophers.

When a substance capable of absorbing moisture is placed in damp air, it becomes gradually more and more humid, until a kind of equilibrium be established; that is, until the affinity of the air be so augmented, and that of the substance be so reduced, that no farther action goes on; and, conversely, when a moist body is placed in dry air, the air abstracts the moisture until a hygrometric equilibrium result. The humidity of the substance becomes thus an index to the dampness of the air in which it is placed.

By taking advantage of this circumstance, and of the changes which certain substances undergo when moistened, several experimenters have attempted to construct hygrometers; but, as yet, no instrument of this kind has been contrived which deserves a higher appellation than hygroscope, or indicator of dampness, and very few of them deserve even this name.

Humidity, while it augments the weights of bodies, is observed also to change their volumes and forms. Thus, a sheet of paper expands very considerably on being moistened, and contracts again on being dried. Hence, in order to stretch a sheet of paper, we first moisten it thoroughly, and then paste it down, or glue down its edges, in order that when dry it may become tense. The varying length of a slip of paper would thus serve to indicate the dampness of the adjacent air, were it not that paper undergoes a permanent change when much moistened, and that there is reason to think that the dust which penetrates its pores tends to alter its hygrometric properties.

The variability of paper renders it very unfit for accurate plans; but no substitute has been found for it. The variation in length from extreme dryness to extreme dampness, is about 1 in 42, but is different in papers of different makes.

Every species of wood is liable to hygrometric changes, a circumstance which gives great trouble in carpentry. The change length-ways is small; but across the grain of the wood the expansion caused by moisture is very great. Generally, the change in the breadth of planks cut radially from the tree is much less than that of planks cut tangentially. This is shown by the manner in which the trunks of trees rend on being dried; the radial fibres contracting much more than the circular ones, and hence producing only radial rents.

On account of this great lateral contraction the panels of doors are fitted into grooves so as to allow of some shrink; for if secured at the edges they would be liable to split.

All organic substances contain pores expressly for the conveyance of their juices, and are influenced by the accession of moisture, some of them very remarkably; and attempts have been made to render the changes which they undergo subservient to the measurement of humidity; but, unfortunately, these changes are very precarious as to their amounts, while repeated fluctuations are found gradually to impair the sensibility of the materials.

The hygrometer of Saussure is formed of a hair from which the oily matter has been removed by a gentle boiling in water containing one per cent. of the sulphate of soda. One end of the hair is fixed by pincers to an adjusting screw, and the other end to the periphery of a small pulley carrying an index, so that a minute change in the length of the hair may produce a perceptible motion in the index; a counterpoise is appended for the purpose of keeping the hair gently stretched.

The whalebone hygrometer of Deluc is constructed on the same principle, but the substance employed is a slip of whalebone cut across the fibres. The instrument is shown in fig. 1. A slender spring keeps the whalebone stretched.

Captain Kater used the twisted Indian grass called Oubeena Hooloo, which possesses the remarkable property of twining and untwining according to the dampness of the air.

The manner of graduating these, and all hygrometers of this class, is to expose them for some time to air thoroughly dried, and then to air thoroughly humid, and to mark the two extremes of the scale, which is usually divided into 100 degrees. For the zero of the scale the air may be dried by means of concentrated sulphuric acid, fused chloride of lime, or any substance having a great affinity for moisture; and for the other extreme it may be moistened by suspending linen rags well soaked in water inside of the receiver, taking the precaution to wet its sides, and to place a saucer of water in it.

None of these instruments, however, give consistent results; so that the whole of them have fallen into disuse as accurate measurers, although they do serve to give a rough idea of the state of the air in regard to moisture.

It is well known that the temperature of a wet body is lower than that of a dry one exposed to the influence of the same air. Leslie applied this depression of temperature to measure the dryness of the air; for this purpose he placed two delicate thermometers near each other, the bulb of the one being covered with moistened linen or paper, and that of the other being dry; the difference between the two was an index to the drying power of the air. The original instrument which he used was composed of two thermometers jointed together, and provided with three slender feet; it is in the possession of the writer of this article. Afterwards Leslie applied the far more delicate differential thermometer (fig. 2) to the same purpose; and his hygrometer is, perhaps, the most delicate instrument which we possess for indicating the humidity of the air. This instrument is formed by uniting two tubes having a ball blown on the end of each, and having a little tinge of sulphuric acid previously introduced. The united tube is then recurved, as shown in the figure, and by a little management the end of the column of acid is brought near to the upper end of one of the tubes. When both balls are at one temperature the instrument indicates zero; and when one of them is covered with wet paper the instrument indicates the depression of temperature. Leslie graduated his hygrometers to show tenths of a centigrade degree.

The indications of this instrument are very precise, and give definite information as to the state of the air; but it is evident that as it merely shows the amount of cold produced by evaporation, it is not strictly a hygrometer. The readings of the thermometer and barometer must also be taken before the quantity of moisture contained in the air can be computed.

It has been proposed, as an improvement on Leslie's hygrometer, to place two thermometers side by side, the bulb of one of them being kept moist by capillary action; this is only a return to the rudier and less delicate form of the instrument.

Warm air is capable of holding in solution more moisture than cold air can, and hence, in order to ascertain the quantity of moisture corresponding to the reading of a Leslie's hygrometer, we must observe the temperature of the air, and deduct from the total quantity of moisture which air at that temperature is capable of holding, the deficiency made apparent by the degree of cooling.

If, then, warm damp air be cooled sufficiently, it may be made to deposit its moisture: as soon as the air reaches that temperature, the moisture which it contains begins to be deposited on the surfaces of surrounding bodies, and dew is formed. Of this we have a familiar example in the dew which is formed on the surface of a jar containing cold water when it is brought into a warm apartment, particularly if the air of the room have been moistened by the breaths of a numerous company; and also in the water which trickles down the insides of our window-panes during a cold night.

Advantage is taken of this circumstance in the construction of Daniell's hygrometer or dew-point instrument. In its first form this hygrometer consisted of a widish tube expanded into a ball at each end; a quantity of ether was introduced previous to the closing of the tube, which was so bent that the whole of the ether might rest in either ball: a small thermometer also was previously inserted, having its bulb half immersed in the ether when in one of the balls. The liquid was then made to boil, so as to expel all the air, and in this state the tube was closed. By this means a Wollaston's cryophorous, on the small scale, was formed.

On bringing all the ether into the one ball, and cooling, by any means, the other ball, the temperature of the ether could be reduced; and if the cooling were carried sufficiently far, dew was formed on the outer surface of the glass. The indication of the inclosed thermometer then gave the dew-point.

The general principle of this hygrometer is excellent; but some little defects in the detail of the operations rendered it somewhat uncertain. In order to produce a sufficient degree of cold, the empty ball was covered with a piece of linen or silk, on which was poured a quantity of ether, and the observer had to watch the instant when the dew began to be deposited on the glass. In this way time could not be allowed for the glass, ether, and thermometer bulb to come all to one temperature, and the indication of the thermometer was therefore influenced by the rapidity or slowness of the cooling.

Dr. Cumming, Professor Connell, and M. Regnault severally made contrivances for remedying this inconvenience; these are all founded on the same principle, and as that of M. Regnault seems best to accomplish the desired effect, we shall confine our observations to it.

M. Regnault's condenser hygrometer (fig. 3), as it is called, consists of a cylinder or thimble, made of very thin silver, and highly polished, 2 inches deep and 1½ inches diameter. This is fitted tightly on a glass tube, cd, open at both ends. The tube has a small lateral tubulure, t, near the upper end. The upper end of the tube is closed by a cork, through the centre of which the stem of a very sensible thermometer is passed. The bulb of the thermometer is in the centre of the silver thimble. A very thin glass tube, fg, open at both ends, passes through the cork, and descends almost to the bottom of the thimble.

Ether is poured into the tube cd as high as to mm, and the tubulure t is placed in communication with an aspirator jar, six or eight pints in capacity, or with a small circular exhausting bellows, by means of the upright supporting pillar H and the flexible tube I, which is furnished with a stop-cock.

On allowing water to run from the aspirator jar, or on opening the bellows, air passes down the tube gf, and rises in bubbles through the ether, which is cooled by the induced evaporation. The refrigeration is the more rapid the more freely the air is made to pass, and the whole mass of ether is of a uniform temperature as it is briskly agitated by the passage of the bubbles of air. The temperature is sufficiently lowered, in less than a minute, to cause an abundant deposition of dew.

By regulating the supply of air, the thermometer may be kept readily at any required temperature less than that of the external air. Now the object of the observer is to discover at what exact temperature the dew begins just to be formed. When, therefore, he has obtained a copious deposition on the bright surface of the silver, he reduces the supply of air so as to allow the thimble to be warmed and the dew to disappear; he then augments the supply of air again to cool the silver cautiously till the dew reappear. In this way he can ascertain the dew-point true to the tenth part of a degree of Fahrenheit's thermometer. The observer must remain at a distance from the apparatus, in order that the warmth of his body, and the humidity of perspiration, may not derange the results; a small telescope or opera-glass may be used to read off the degrees.

Less ether is needed with this instrument than with the original form of Daniell's hygrometer, and, unless when the air is very dry, the ether may be replaced by alcohol.

In order to measure the temperature of the air, another apparatus exactly similar, but containing no ether, is placed under the influence of the aspirator.

These two, the wet-bulb and the dew-point hygrometers, are the only instruments which have yet been contrived for giving definite information as to the humidity of the atmosphere; but neither of them indicates per se the quantity of moisture contained in a given volume of air; that quantity can only be ascertained by help of carefully conducted experiments.

Our knowledge of this branch of the subject is extremely limited, and the attention of scientific men is at present directed to the law which connects the indications of the dew-point and wet-bulb hygrometers.

For this purpose the simultaneous readings of the two instruments, of the thermometer and barometer, are all observed under as great a variety of circumstances as possible. It is to be regretted, however, that in making these comparisons an attempt is made to discover the ratio between the depressions of the dew-point and of the wet-bulb below the temperature of the air; a formula of the form

\[ T - t = f(T - t) \]

being used, in which \( T \) is the temperature of the air, \( t \) that of the dew-point, and \( t' \) that of the wet-bulb; \( f \) being a coefficient, the values of which corresponding to different values of \( T \) are sought to be ascertained. Now one may easily see that when the air is nearly saturated with moisture both of the depressions \( T - t \) and \( T - t' \) must be small; but it does not follow that they are proportional to each other; for if we imagine the air to be nearly dry, the wet-bulb hygrometer will be nearly at its maximum reading, while the dew-point instrument has, so to speak, no maximum, since, when the air is perfectly dry, no conceivable lowering of temperature can ever cause it to deposit moisture.

The discussion of these matters, however, belong to the subjects of Meteorology and Evaporation, to which articles the reader is referred for further information. (E. S.)