CHAPTER XI.

INSTRUMENTS FOR ASCERTAINING THE HUMIDITY OF THE AIR.

=97. Hygrometric Substances.=--The instruments devised for the purpose of ascertaining the humidity of the atmosphere are termed _hygrometers_. The earliest invented hygrometers were constructed of substances readily acted upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c., which all absorb moisture, and thereby increase in length, and when deprived of it by drying they contract. Toy-like hygrometers, upon the principle of absorption, are still common as ornaments for mantel-pieces. A useful little instrument of this class, formed from the beard of the wild oat, is made to resemble a watch in external appearance, and is designed to prove the dampness or dryness of beds: a moveable hand points out on the dial the hygrometric condition of the clothes upon which the instrument is laid.

=98. Saussure's Hygrometer=, formerly used as a meteorologic instrument, but now regarded as an ornamental curiosity, is represented in fig. 75. Its action depends upon a prepared hair, fixed at one end to the frame of the instrument, and wound round a pulley at the other. The pulley carries a pointer which has a counterpoise sufficient to keep the hair stretched. By this means the shrinking and lengthening of the hair cause the pointer to traverse a graduated arc indicating the relative humidity.

Such instruments, however ingenious, are not of scientific value; because they do not admit of rigid comparison, are liable to alter in their contractile and expansive properties, and cannot be made to indicate precisely alike.

=99. Dew-Point.=--The amount of water which the air can sustain in an invisible form increases with the temperature; but for every definite temperature there is a limit to the amount of vapour which can be thus diffused. When the air is cooled, the vapour present may be more than it can sustain; part will then be condensed as dew, rain, hail or snow, according to the meteorologic circumstances. The temperature which the air has when it is so fully saturated with vapour that any excess will be deposited as dew, is called the _dew-point_.

=100. Drosometer.=--"To measure the quantity of dew deposited each night, an instrument is used called a _Drosometer_. The most simple process consists in exposing to the open air bodies whose exact weight is known, and then weighing them afresh after they are covered with dew. According to Wells, locks of wool, weighing about eight grains, are to be preferred, which are to be divided [formed] into spherical masses of the diameter of about two inches."--_Koemtz._

=101. Humidity.=--The proportion existing between the amount of vapour actually present in the air at any time, and the quantity necessary to completely saturate it, is called _the degree of humidity_. It is usually expressed in a centesimal scale, 0 being perfect dryness, and 100 complete saturation.

The pressure, or tension, of vapour at the dew-point temperature, divided by the tension of vapour at the air temperature and the quotient multiplied by 100, gives the degree of humidity. (Regnault's Tables should be used.)

Hence the utility of instruments for determining the dew-point.

=102. Leslie's Hygrometer.=--This instrument consists of a glass syphon tube, terminated with a bulb or ball at each end, turned outwards from each other, as in fig. 76. The tube is partly filled with concentrated sulphuric acid, tinged by carmine. One of the balls is covered smoothly with fine muslin, and is kept continually moistened with pure water, drawn from a vase placed near it by the capillary attraction of a few strands of clean cotton-wick. The descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. The drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. When the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air.

"Should the water become frozen on the ball, this hygrometer will still act; for evaporation goes on from the surface of ice in proportion to the dryness of the air. Leslie estimates, that when the ball is moist, air, at the temperature of the ball, will take up moisture equal to the sixteen-thousandth part of its weight, for each degree of his hygrometer; and as ice in melting requires one-seventh of the caloric consumed in converting water into vapour, when the ball is frozen, the hygrometer will sink more than when wet by 1 deg. in 7 deg.; and hence, in the frozen state, we must increase the value of the degrees one-seventh: so that each of them will correspond to an absorption of moisture equal to one-fourteen-thousandth part of the weight of the air.

"When this hygrometer stands at 15 deg., the air feels damp; from 30 deg. to 40 deg., we reckon it dry; from 50 deg. to 60 deg., very dry; and from 70 deg. upwards, we should call it intensely dry. A room would feel uncomfortable, and would probably be unwholesome, if the instrument in it did not reach 30 deg.[8] In thick fogs it keeps almost at the beginning of the scale. In winter, in our climate, it ranges from 5 deg. to 15 deg.; in summer often from 15 deg. to 55 deg.; and sometimes attains 80 deg. or 90 deg. The greatest degree of dryness ever noticed by Leslie was at Paris, in the month of September, when the hygrometer indicated 120 deg."--_Professor Trail, in "Library of Useful Knowledge."_

In estimating the value of the indications of this hygrometer, it should be borne in mind that the scale adopted by Leslie was _millesimal_, that is to say, from the freezing to the boiling-point of water was divided into a thousand parts; ten millesimal degrees are therefore equal to one of the scale of Celsius.

103. DANIEL'S HYGROMETER.

This instrument was invented about the year 1820, by Professor Daniel, the distinguished author of _Meteorological Essays_; and it entirely superseded all hygrometers depending upon the absorption of moisture. The form of the instrument is shown in fig. 77.

It consists of a glass tube, about one-eighth of an inch in diameter of bore, bent twice at right angles, and terminated, at each end, in a bulb about one inch and a quarter in diameter. In one limb of the tube is enclosed a delicate thermometer, which descends to the centre of the adjoining bulb, which is about three-parts filled with sulphuric ether. All the other parts of the tube are carefully freed from air, so that they are occupied by the vapour of the ether. This bulb is generally made of black glass; the other is transparent, but covered with a piece of fine muslin. The support for the tube has a thermometer attached, which shows the temperature of the external air. The tube can be removed from the stand, and the parts are made to pack, with a necessary phial of ether, in a small box, which can easily be got into the pocket.

_How to use the Hygrometer._--This instrument gives the dew-point by direct observation, which must be made in the following manner:--Having fixed the tube upon the stand, with the bulbs vertically downward, the ether is all caused to flow into the lower ball by inclining the tube. The temperature of the air is noted by the exposed thermometer. Then some ether is poured, from a dropping tube fitting into the neck of the phial, upon the muslin-covered bulb. The rapid evaporation of this ether cools the bulb and causes condensation of the ethereal vapour in its interior. This gives rise to rapid evaporation of the ether in the lower bulb, whereby its temperature is greatly reduced. The air in the vicinity is deprived of its warmth by the cold bulb, and is soon cooled to the temperature at which it is perfectly saturated with the vapour which it contains. Cooled ever so little below this temperature, some aqueous vapour will be condensed, and will form a dew upon the black-glass bulb. At the first indication of the deposit of dew the reading of the internal thermometer is taken: which is the dew-point.

This hygrometer has undeniable disadvantages. The surface upon which the dew condenses is small, and requires a peculiar direction of light in which to see it well. The observer, having his attention on the bulb and the thermometer, cannot always fix with precision the dew-point; and hence he is recommended to note the temperature at the appearance and at the disappearance of the dew, in order that the chance of error may be diminished. Without doubt, the necessarily long continuance of the observer near the instrument influences, to some extent, the observed temperatures; and the difficulty of not being always able to procure pure ether for the experiments is not the least of the drawbacks to the use of the instrument. Some of these disadvantages are obviated in Regnault's hygrometer.

104. REGNAULT'S CONDENSER HYGROMETER

(Fig. 78) consists of a tube, _C_, made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1.8 inches in depth, by 0.8 in diameter; this is fitted tightly to a brass stand, _B_, with a telescopic arrangement for adjusting when making an observation.

The tube, _C_, has a small lateral tubulure, to which is attached an India-rubber tube, with ivory mouth-piece; this tubulure enters _C_ at right angles near the top, and traverses it to the bottom of the largest part.

A delicate thermometer, _D_, is inserted through a cork, or India-rubber washer, at the open end of the tube, _C_, the bulb of which descends to the centre of its largest part.

_G_ is an attached thermometer for taking the temperature of the air, and _F_ is a bottle containing ether.

_To use the Condenser Hygrometer_, a sufficient quantity of ether is poured into the silver tube to cover the thermometer bulb: on allowing air to pass bubble by bubble through the ether, by breathing in the tube, _E_, an uniform temperature will be obtained; if the ether continues to be agitated, by breathing briskly through the tube a rapid reduction of temperature will be the result; at the moment the ether is cooled down to the dew-point temperature, the external surface of that portion of the silver tube containing ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point.

This form of hygrometer, for ascertaining by direct observation the dew-point, is so superior to Daniell's, both from its being more certain in its indications and economical in use, that Messrs. Negretti and Zambra have been induced to modify it, and reduce its price to little more than that of a good Daniell's Hygrometer.

=105. Temperature of Evaporation.=--When the air is not saturated with vapour, evaporation is going on with more or less activity, according as the temperature is high or low, rising or falling. Now vapour cannot be formed without an expenditure of heat; as we invariably find that the process of evaporation lowers the temperature of the liquid from which the vapour is produced, and, by communication, that of contiguous substances also. Thus the emigrant, crossing the line under the scorching influence of the vertical sun, wraps a wet towel round his can of water, swings it in the breeze, to evaporate the moisture of the towel, and obtains a glass of cool water. So also, European residents in India, during the hot season, spread out mats in their apartments, and keep them wet, in order that the evaporation may cool the air. This principle has been applied, for the purpose of ascertaining the hygrometric condition of the air, in the instrument known as Mason's hygrometer, or psychrometer, which is now in general use, from its simplicity, accuracy, and ease of observing.

106. MASON'S HYGROMETER.

=The Dry and Wet Bulb Hygrometer, or Psychrometer=, known also as Mason's hygrometer (fig. 79), consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one marked _dry_, the other _wet_. The bulb of the wet thermometer is covered with thin muslin, and round the neck is twisted a conducting thread of lamp-wick, which passes into a vessel of water, placed at such a distance as to allow a length of conducting thread, of about three inches; the cup or glass is placed on one side, and a little beneath, so that the water within may not affect the reading of the _dry bulb thermometer_. In observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation.

The _dry_ bulb thermometer indicates the temperature of the air itself; while the wet bulb, cooled by evaporation, shows a lower temperature according to the rapidity of evaporation.

_To find the Dew-point._--From the readings of the two thermometers, the dew-point can be deduced by formulae (that known as Apjohn's is considered the most theoretically true), or from the valuable Hygrometric Tables by J. Glaisher, Esq., F.R.S.

For practical purposes in estimating the comparative humidity, the annexed table, which is a reduction from Mr. Glaisher's elaborate work, will be sufficient; it will at least serve to assist in familiarising the inexperienced in the value of the psychrometer's indications:--

+------------------------------------------+ | | Difference between Dry-bulb | | | and Wet-bulb Readings. | |Temperature |-----------------------------| | by the | 2 deg. | 4 deg. | 6 deg. | 8 deg. | 10 deg.| 12 deg.| | Dry Bulb |-----------------------------| |Thermometer.| Degree of Humidity. | |------------------------------------------| | 34 deg. | 79 | 63 | 50 | .. | .. | .. | | 36 | 82 | 66 | 53 | .. | .. | .. | | 38 | 83 | 68 | 56 | 45 | .. | .. | | 40 | 84 | 70 | 58 | 47 | .. | .. | | 42 | 84 | 71 | 59 | 49 | .. | .. | | 44 | 85 | 72 | 60 | 50 | .. | .. | | 46 | 86 | 73 | 61 | 51 | .. | .. | | 48 | 86 | 73 | 62 | 52 | 44 | .. | | 50 | 86 | 74 | 63 | 53 | 45 | .. | | 52 | 86 | 74 | 64 | 54 | 46 | .. | | 54 | 86 | 74 | 64 | 55 | 47 | .. | | 56 | 87 | 75 | 65 | 56 | 48 | .. | | 58 | 87 | 76 | 66 | 57 | 49 | .. | | 60 | 88 | 76 | 66 | 58 | 50 | 43 | | 62 | 88 | 77 | 67 | 58 | 50 | 44 | | 64 | 88 | 77 | 67 | 59 | 51 | 45 | | 66 | 88 | 78 | 68 | 60 | 52 | 45 | | 68 | 88 | 78 | 68 | 60 | 52 | 46 | | 70 | 88 | 78 | 69 | 61 | 53 | 47 | | 72 | 89 | 79 | 69 | 61 | 54 | 48 | | 74 | 89 | 79 | 70 | 62 | 55 | 48 | | 76 | 89 | 79 | 71 | 63 | 55 | 49 | | 78 | 89 | 79 | 71 | 63 | 56 | 50 | | 80 | 90 | 80 | 71 | 63 | 56 | 50 | | 82 | 90 | 80 | 72 | 64 | 57 | 51 | | 84 | 90 | 80 | 72 | 64 | 57 | 51 | | 86 | 90 | 80 | 72 | 64 | 58 | 52 | +------------------------------------------+

The total quantity of aqueous vapour which at any temperature can be diffused in the air being represented by 100, the per-centage of vapour actually present will be found in the table opposite the temperature of the dry thermometer, and under the difference between the dry-bulb and wet-bulb temperatures. The degree of humidity for intermediate temperatures and differences to those given in the table can be easily estimated sufficiently accurately for most practical purposes.

The difference between the two thermometer readings taken from the reading of the wet bulb, gives the dew-point very nearly, when the air is at any temperature between freezing and 80 deg. This simple rule will be found serviceable to horticulturists, since it will enable them to estimate the chilling effect of dew or hoar-frost on tender plants.

_Use as an Indicator of Weather._--In our climate, the usual difference between the thermometer readings,--in the open air, shaded from the sun, reflected heat, and currents of air,--ranges from one to twelve degrees. In hot and dry climates, as India and Australia, the range out of doors has been found as much as 30 deg., occasionally.

When the moisture is frozen, the bulb should be wetted afresh, and the reading taken just before it again freezes; but the observation then is of little value, and for general purposes need not be taken, as the air is known to be dry in frosty weather.

The muslin or cotton rag should be washed once or twice a week by pouring water over the bulb; and it should be replaced by a fresh piece at least once a month. Accuracy depends very much upon keeping the wet bulb clean, and not _too_ wet.

In connection with the barometer, this hygrometer is very useful, not only on land, but especially at sea, where other kinds of hygrometers cannot be practically used. A fall in the barometer is indicative of coming wind or rain: if the hygrometer shows increasing dampness by the difference of the readings becoming smaller,--rain may therefore be anticipated. On the contrary, if the hygrometer shows continuing or increasing dryness, a stronger wind is probable, without rain.

_Domestic Uses._--Mason's hygrometer is useful in regulating the moisture of the air of apartments; a difference in the thermometer readings of from 5 deg. to 8 deg. being considered healthy. Many complaints require that the temperature and humidity of the air which the invalid breathes should be carefully regulated. Hence it is a valuable household instrument. In a room, it should be placed away from the fire as much as possible, but not exposed to draughts of air.

Figs. 80 and 81 show cheap arrangements of the instrument for domestic purposes. Other arrangements are given to the instrument to make it suitable for exhibiting the hygrometrical state of the air in hot-houses, conservatories, malting-houses, warehouses, manufactories, &c.

Fig. 82 shows the instrument arranged on brass tripod stand, with folding legs and metal cover, to render it portable.

=107. Self-Registering Hygrometer.=--A maximum thermometer and a minimum thermometer, each fitted up as a wet-bulb thermometer, record the highest and lowest temperature of evaporation during the interval of observation. Negretti's mercurial maximum, and an alcohol minimum, answer best.

=108. Causes of Dew.=--"The aqueous vapour of our atmosphere is a powerful radiant; but it is diffused through air which usually exceeds its own mass more than one hundred times. Not only, then, its own heat, but the heat of the large quantity of air which surrounds it, must be discharged by the vapour, before it can sink to its point of condensation. The retardation of chilling due to this cause enables good solid radiators, at the earth's surface, to outstrip the vapour in their speed of refrigeration; and hence, upon these bodies, aqueous vapour may be condensed to liquid, or even congealed to hoar-frost, while at a few feet above the surface it still maintains its gaseous state."[9] The amount of moisture so deposited will vary with different atmospheric conditions. If the sky be decidedly cloudy or misty, the heat radiated from the earth will be partly restored by counter-radiation from the visible vapour; the cooling of the earth's surface will, therefore, take place slowly, and little dew will be deposited. On the other hand, if the air contain transparent vapour, and the sky appear clear, the counter-radiation will be less, the earth will cool rapidly, and the deposit of dew will be copious; provided the night be comparatively calm, for, when the wind blows, the circulating air supplies heat to the radiating substances, and prevents any considerable chilling.

The dew which falls in tropical countries greatly exceeds in abundance what we experience in our climate; because the air is there, from the great heat, capable of sustaining a large amount of vapour in the transparent state, and the conditions most favourable for a maximum reduction of temperature by radiation are present. At those places, or upon those substances which cool the lowest and most readily, the dew falls most copiously.

=109. Plan of Exposing Thermometers=, &c.--Figure 83 is an illustration of a convenient slab for supporting thermometers in an exposed position attached to a stand (such as Glaisher's, described in Chapter XVI.) for ordinary scientific observations. It has a projecting ledge, _B_, to carry off rain from the instruments, the slab, _A_, being erected vertically. The hygrometer is placed at _E_, with the vase of water at _F_. An alcohol minimum thermometer is represented at _C_, in the position most favourable to its certain action; and at _D_ is shown one of Negretti & Zambra's maximum thermometers, the position of which may be more nearly horizontal than there exhibited, although a slight depression of the bulb-end of the frame is desirable, but not necessary, as this thermometer can be used in any position.