Hy′grome-ter.
An instrument for measuring the comparative moisture of the air.
They are of three kinds : —
1. Those which act on the principle of
absorption.
2. Those which act by
condensation.
3. Those in which the hygrometric condition is deduced from observations of a wet and a dry bulb.
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Hygrometers |
1.
a. Of the first class is the
hygrometer of
Saussure (died 1799). It consists of a human hair boiled in lye, and acts by absorption and evaporation, as shown at
a,
Fig. 2628. A piece of catgut may be made to extend from a pin at one end, over a number of pulleys, and terminate at the lower end with a small weight, which rises and falls with changes in the length of the cord, induced by hygrometric changes in the atmosphere.
A hygrometer, or
weather prophet, may be constructed by taking a cigar-box and inserting at its center an upright pivoted shaft with two horizontal arms.
Around this are wound two turns of a catgut string, one end of which is fastened to a staple at one end and the other to a spiral spring at the other end of the box.
Fluctuations in atmospheric humidity cause the string to extend or contract, as the case may be, turning the shaft partially round.
If to the end of one arm a toy figure of a mower be attached, and to the other a figure holding an umbrella, openings being cut to represent doors in the side of the box, fronting each figure, the figure holding the umbrella will come out when the air is moist and retire as the weather becomes dry, allowing the mower to come forth.
In
b, a human hair is fastened at the upper end to a screw, and at the lower end passes over a pulley which has a radial pointer.
A weight keeps the hair tight, and its expansion or contraction causes the pointer to traverse on the graduated are. Moisture makes the hair longer, and allows the pointer to rise, and dry weather has the reverse effect.
The 0° and 100° form the extremes of dryness and wetness: one is obtained from an atmosphere artificially dried, and the other by the absolute saturation of the air with moisture.
The intervening space is then divided into 100 equal parts.
b. An alcoholic solution of camphor, saltpeter, and muriate of ammonia, in a glass tube, has been suggested for a weather guide.
Changes of weather are stated to render the solution turbid, or clear it, as the condition may be. Its value is doubtful.
c. An arm is balanced upon an axis so as to be horizontal at the zero point, and provided at one end with an absorbent substance, such as a piece of sponge.
When the air is relatively moist the sponge will absorb water and become heavier, so that the pointer rises against the graduated arc and indicates rain; conversely, dry air causes the sponge to part with some of its moisture, and, becoming lighter, the pointer falls.
2.
Daniell's hygrometer
c determines the moisture of the air by indicating the dew-point, or the temperature at which moisture is deposited by the air. It consists of a bent tube of glass with a bulb at each end. The legs of the tube are of unequal length, and the lower one, which is of black glass, contains a little ether, into which dips the bulb of a small and delicate thermometer, whose stem occupies the cavity in the leg of the tube.
The whole tube contains ether and its vapor, the air having been removed.
The upper bulb is covered with a piece of muslin.
The support of the instrument has another thermometer by which the temperature of the air is denoted.
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When an observation is to be made, a little ether is poured upon the muslin; this evaporates and cools the contents of the tube, abstracting heat from the lower bulb by the vaporization of its contents.
As soon as the lower bulb has cooled sufficiently to condense the moisture of the atmosphere, dew will be observed to condense upon it, and the temperature is to be noted as indicated by the thermometer in the tube.
If the air be moist, dew will commence to deposit on the lower bulb on a slight reduction of temperature; if the air be dry, it will require a relatively greater reduction of temperature.
The point at which moisture commences to form on the bulb is the “dew-point,” and this ascertained, in connection with the temperature of the air, will enable the hygrometric condition of the air to be determined by tables prepared for the purpose; as to the elasticity and density of the aqueous vapor, its weight in a cubic foot of air; the degree of dryness either upon the scale of the thermometer or the hygrometer, and the rate of evaporation.
In the climate of
England the dew-point is seldom more than 30° below the temperature of the air, but in the Deccan, in
India, with the temperature of the air at 90°, the dew-point has been seen at 29°, — making the degree of dryness 61° thermometric.
Professor Daniell states that
the more accurate mode of expressing the moisture of the air from an observation of the temperature and dew-point is by the quotient of the division of the elasticity of the vapors at the real atmospheric temperature, by the elasticity at the temperature of the dew-point; for, calling the term of saturation 1,000, as the elasticity of vapor at the temperature of the air is to the elasticity of vapor at the temperature of the dewpoint, so is the term of saturation to the observed degree of moisture.
Thus with regard to the observation in the Deccan — 1.430 : 0.194 : 1000 : 135, — the fourth term is the degree of moisture on the hygrometric scale.
3.
d c are two different forms of
Mason's hygrometer.
In this two thermometers are placed side by side, the bulb of one being covered with muslin or similar material, and wetted with water when an observation is to be made.
The dimished temperature of the wet bulb, due to the evaporation of the moisture, is compared with the natural temperature of the other bulb; the temperature and the difference afford the data for determining the hygrometric conditions.
In
e the bulb of one thermometer is kept constantly moist from a cistern fixed on the plate, to which the two are attached.
This is the most useful form of hygrometer.
The theoretical relation between the indications of the two bulbs and the humidity of the air is rather complex and has given rise to much debate; it is usual to effect the reduction by tables which have been empirically constructed by comparison with the indications of the
dew-point instrument of
Daniell.
The British tables for this purpose were constructed by
Glaisher; edition of 1856 preferred.
They are based upon a comparison of the simultaneous readings of the wet and dry bulb thermometers, and of
Daniell's hygrometer, taken for a series of years in Greenwich Observatory, in
Toronto, and in
India.
The ratio between the two readings is not equal at all degrees of temperature as marked on the dry bulb.
When this temperature — the
natural, it may be called — is 53° F. the
dew-point is as much below the
wet bulb as the latter is below 53° F., the temperature of the air. At higher temperatures the wet-bulb reading is nearer to the dew-point than to the air-temperature, and the reverse is the case at temperatures below 53° F. See psychrometer.
Two citations from “Cosmos” are not irrelevant: “In the brilliant period of the foundation of ‘mathematical natural philosophy,’ attempts to investigate the moisture of the atmosphere in its connection with variations of temperature and with the direction of the wind were not wanting.
The Academia del Cimento conceived the happy idea of determining the quantity of vapor by evaporation and precipitation.
The oldest Florentine hygrometer was accordingly an apparatus in which the quantity of precipitated water run off was determined by weight.
To this condensation hygrometer, which, aided by the ideas of
LeRoy, has gradually led in our own days to the exact psychrometric methods of
Dalton,
Daniell, and Auguste, there were added, according to the example previously set by Leonardo da
Vinci, the absorption hygrometers, made of animal or vegetable substances, of
Santorio (1625), Torricelli (1626), and
Molyneux.
Catgut and the beard of the wild oat were used almost at the same time.
Instruments of this kind, founded on the absorption of the aqueous vapor contained in the atmosphere by organic substances, were provided with indexes and counterpoises, and were very similar in construction to
Saussure's and Delue's hair and whalebone hygrometers; but the instruments of the seventeenth century were deficient in the determination of fixed wet and dry points, so necessary for the comparison and understanding of the results.
This desideratum was at last supplied by
Regnault, but without reference to the variation which might be occasioned by time in the susceptibility of the hygrometic substances employed.
Pictet, however, found that the hair of a Guanche mummy from
Teneriffe, which might be a thousand years old, employed in a
Saussure's hygrometer, still posseessed a satisfactory degree of sensibility.”
“ ‘The admiral’ [Columbus], says
Fernando Colon, ascribed the many refreshing falls of rain which cooled the air whilst he was sailing along the coast of
Jamaica, to the extent and density of the forests which clothe the mountains.
He takes this opportunity of remarking, in his ship's journal, that ‘formerly there was much rain in
Madeira, the Canaries, and the
Azores; but since the trees which shaded the ground have been cut down, rain has become much more rare.’
This warning has remained almost unheeded for more than three centuries and a half.”
