the water in the tube soon rises in temperature and the cloud disappears, the moment of its disappearance being that when the dew-point is again reached.

The operation may be repeated as long as the water in the vase remains at a temperature below the dew-point. This form of hygrometer is very well suited for use in any rooms where it is required to regulate the dampness of the air very carefully.

Indirect Hygrometers.-We shall first treat of those which are organic. These all depend on the wellknown property of organic bodies to alter their molecular arrangement or their appearance according to their hygrometrical condition. We all know that ropes become shortened by wet, but hair is affected in the opposite way, and grows longer when it grows damp. Saussure was the first to adapt this principle to scientific uses.

The essential part of his instrument-the Hair

Hygrometer-is a human hair which has never been oiled or roughly handled. Such a hair stretches as it grows damp and contracts as it dries. It is kept fixed at one end and stretched by a light weight at the other, the cord of connection passing round the sheaf of a block, to which is attached an arm moving on a graduated arc. The position of the arm on the arc, therefore, depends on the length of the hair from its point of suspension above.

It is obvious that, though such an arrangement can be made to indicate the amount of moisture in the air, it cannot afford an absolute measure of that moisture, inasmuch as the normal length which the hair possesses, when the atmosphere is charged with moisture, must be determined for each hair individually. This is effected by observing the instrument at a time of perfect saturation of the air, produced either naturally or artificially, and moving the upper screw till the index. is brought exactly to 100-the point of perfect saturation. The instrument is then fit for use.

I have explained the principle of the hair hygrometer at some length, as, although it is seldom used in this country, it affords practically the most satisfactory means of determining the hygrometrical condition of the air at temperatures close to and below the freezingpoint, under which circumstances the dry-and-wet-bulb hygrometer fails.

Numerous other attempts have been made to utilise organic substances for hygrometrical observations, but none of them have resulted in the production of a useful scientific instrument. As an instance we may cite the use of seaweed, which, owing to the hygroscopic properties of the salts which it contains, grows

damp in wet weather and dries when the weather clears up. The well-known weather-house toy, in which a woman comes out when the air is dry and a man when it is damp, depends on the expansion and contraction of a piece of catgut, according to the hygrometrical condition of the atmosphere.

FIG. 22.

Of Indirect Inorganic Hygrometers the only one to be noticed is the dry-and-wet-bulb hygrometer, first proposed by Hutton in Edinburgh,' and subsequently by Mason in this country. This is the apparatus almost universally employed at present, and consists of two thermometers, of which one. has its bulb coated with muslin (fig. 22) and kept moistened with water. The principle of the instrument is that as long as the atmosphere is not saturated with moisture evaporation will take place from any damp surface exposed to it, such as the moist coating of the wet bulb. If the air be saturated no evaporation is possible, and the two thermometers will read alike; but if it be not saturated the temperature of the coated bulb will fall until it reaches a certain point intermediate between the temperature of the dry-bulb thermometer and of the dew-point. Once that limit is

Wet Bulb
Mounting.

1 Trans. R. S. Edin., vol. v. pt. 3, 'History of the Society.' Life of Hutton, by Playfair, p. 67. To one who considers meteorology with attention, the want of an accurate hygrometer can never fail to be a subject of regret. The way of supplying this deficiency which Dr. Hutton practised was by moistening the ball of a thermometer, and observing the degree of cold produced by the evaporation of the moisture. The degree of cold, cæteris paribus, will be proportional to the dryness of the air, and affords, of course, a measure of that dryness. The same contrivance, but without any communication whatsoever, occurred afterwards to Mr. Leslie' (Sir John Leslie).

reached, if the supply of water be kept constant, the wet bulb will not change its indication unless the actual amount of moisture in the air varies.

The supply of water to the wet bulb is effected either by moistening it with a wet brush, or, for continuous observations, by wrapping a few threads round the neck of the bulb (fig. 22), and allowing them to dip into a water-vessel placed at a distance of a few inches. Such an arrangement must fail in frost, as then capillary action along the threads will cease; and also care must be taken to make the supply more copious in very dry weather than at other times. Full directions for managing the wet-bulb thermometer will be found in the Instructions' already quoted.

The theory of the instrument has been investigated in Germany by August and in this country by Professor Apjohn (Transactions of the Royal Irish Academy,' vol. xvii.), who gives the following equations :

much from unity, at stations situated near the sea level, and the formula is commonly abbreviated thus:

For temperatures below the freezing-point this formula becomes

=

*Where f" the tension of aqueons vapour existing in the air, the temperature corresponding to which is the dew-point; f'the tension of vapour corresponding to the reading of the wet bulb; t=the temperature of the dry bulb; t'the temperature of the wet bulb; p= the reading of the barometer.

These formulæ, strictly speaking, require the use of tables of the tension of aqueous vapour, but, for convenience' sake, special tables have been constructed, giving the dew-point and other hygrometrical results by inspection. The tables most generally used in this country are Glaisher's,' and are based on a series of numbers called the Greenwich Factors.

These factors were determined by comparisons between simultaneous observations of the dry and wet bulb hygrometer and Daniell's hygrometer, made at Greenwich, with similar comparisons of observations taken at high temperatures in India, and others made at low and medium temperatures at Toronto. The tables are fairly accurate, for the conditions and range of humidity which usually occur in the neighbourhood of London; but they are insufficient for the conditions. of great dryness which occasionally are noticed here, and, à fortiori, they fail in the extreme aridity of the climate of India, or in the intense cold which occurs, for instance, in Canada. That this must be so is evident when we point out that the greatest range of temperature for which provision is made in the factors is from 10° to 100°, and that the greatest depression of the wet bulb, below the dry, allowed for in the tables is 24-limits insufficient to meet the requirements of such climates as have been mentioned.

The greatest objection to the wet bulb hygrometer is the difficulty of managing it in frost. Although a formula has just been given for readings below 32°, it is evident, from the sudden change in the denominator

1 Hygrometrical Tables, adapted to the use of the Dry and Wet Bulb Thermometer. By James Glaisher, F.R.S. London: Taylor & Francis, 6th edition. 1870.