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to the productive quality of the soil, but also raises the temperature of the air around it. For moss can retain five times its own weight of water on the surface of the soil always prepared for evaporation, and prepared in the manner that it can most readily evaporate. No one need be astonished at the coldness of our hills and the dampness of our mountain climate, when they consider that the entire surface of the soil is either covered with moss or grassy turf. This moss or turf retains most of the water that falls from the clouds, permitting little, except in very wet weather, to enter the soil; this water is almost immediately evaporated, and the turf prepared to receive other supplies. The coldness caused by evaporation must therefore be very great." It follows from this, that the conversion of a swamp or a low-lying damp piece of ground into a lake will add materially to the dryness and amenity of the climate of the surrounding district; and the rainier the locality the greater will be the advantage gained.
319. The Hygrometry of the Atmosphere. - At all temperatures, even the lowest, moisture exists in the atmosphere in an invisible state, so that the air is never absolutely dry. Intervals occur between the particles of air which are partially filled with the vapour that is constantly rising from the earth. This property is termed the capacity of the air for moisture, and when the intervals between the particles are full of vapour, the air is said to be saturated. Professor J. D. Everett constructed a hygrometer (Dry-and-Wet-bulb) with the degrees on a large scale and carefully compared with a standard, and found that instances of exact correspondence of the two thermometers, which happen when the air is perfectly saturated, are events of as rare occurrence as eclipses of the sun or moon. Thus, then, though every time dew is formed the air in contact with the ground must be brought to the point of saturation, and every time cloud is formed and rain falls the point of saturation is also reached where the condensation takes place, yet the air a few feet above the ground is seldom perfectly saturated.
320. An increase of temperature, by expanding the air and
thus separating the particles farther from each other, increases the capacity of the air for moisture. On the other hand, a fall of temperature, by drawing the particles closer together, diminishes the capacity. But the capacity of the air for moisture increases at a more rapid rate than the temperature. Thus, air can contain at 32° the 160th part of its own weight, at 59° the 80th part, and at 86° the 40th part; the law being that for every increase of 27° the capacity is doubled.
321. Hygrometer. — The instrument for ascertaining the amount of vapour in the atmosphere is called a hygrometer. There is a great variety of hygrometers, differing both in form and in principle of construction. Some are formed of substances which, by readily absorbing moisture from the air, and as readily parting with it, change in form and size, and thus give some indication of theamount of vapour present in the air. Of these the most noteworthy is the hair hygrometer of Saussure, which, when
Fig. 24. the air is damp, absorbs moisture and becomes shorter, and, when the air is dry, returns to its original length. The conservatory hygrometer (fig. 24), constructed by Richard Adie, belongs to this class. The pointer is made of two pieces of wood glued together, so that increasing dampness twists it to the right, and increasing dryness to the left. Though of little or no scientific value, yet it may be turned to good account in the sick-room or in conservatories.
322. It is this property of substances to be changed in bulk in attracting moisture from the atmosphere, or in parting with it, which explains a large number of popular prognostics of the weather, especially such as refer to the feelings and con
duct of animals, the opening and closing of flowers, and the lengthening and shortening of strings, cordage, and other materials.
323. Hygrometers constructed on the principle of absorption are faulty, not only because they are irregular in their action, but also because in the course of time they are subject to great change. The most accurate hygrometers are those constructed on the principles of condensation or of evaporation. A familiar illustration of the principle of condensation is the forming of dew on a tumbler filled with cold water on being brought into a warm room. This dew is caused by the deposition of moisture from the air, which, in contact with the cold surface of the glass, is now cooled down below the point of saturation. The temperature of the glass at the moment dew begins to form on its surface is termed the dew-point, which corresponds with the point of saturation.
324. Daniell's and Regnault's hygrometers are constructed on the principle of this simple phenomenon, having only
superadded to them various contrivances for reducing the temperature quickly to any point that may be desired, and for observing the temperature at which dew is formed with precision. Daniell's hygrometer (fig. 25) consists of a glass tube bent at right angles at two points, with a bulb at each extremity. One of the bulbs is nearly filled with ether, into which is plunged the ball of a delicate thermometer.
The other bulb is covered with Fig. 25.
muslin; this being wetted with a
few drops of ether, evaporation of the ether takes place, which quickly cools the bulb, and thus condenses the vapour of the ether which is within. In consequence of this the ether inside the other bulb evaporates,
and its temperature being thereby reduced, a ring of dew begins to be formed outside the bulb. At this instant the ther-, mometer inside is read, and the reading gives the dew-point of the air at the time. At the same time the temperature of the air is given by the thermometer which is freely exposed to the air on the upright stand. This is the simplest and cheapest of the two hygrometers, but Regnault's requires less time in making the observations. Owing to the trouble and expense attending the use of all hygrometers which give the dewpoint directly by condensation, another hygrometer has come into general use by which the dewpoint may be determined indi
0 rectly by evaporation-viz., the dry-and-wet-bulb thermometers.
325. This hygrometer (fig. 26) consists of two mercurial thermometers, which, being placed side by side, would indicate the same temperature.
The drybulb is a common thermometer, intended to show the temperature of the air. The wet-bulb is also a common thermometer, having its bulb covered with a piece of thin muslin, from which pass a few threads of darning cotton or narrow strip of muslin into a small vessel containing rain-water. Water rises by capillary attraction from the vessel to the muslin, thus keeping it
Fig. 26. constantly wet. When the air is dry, evaporation proceeds rapidly from the muslin, and on account of the heat lost by evaporation the wet-bulb indicates a lower temperature than the dry-bulb. But when the air is
damp, evaporation is slower, and the difference between the two thermometers becomes smaller; and when the air is completely saturated evaporation ceases, and the two thermometers indicate the same temperature.
326. Some precaution is required in taking the observations when the temperature of the air is below 32°. In such cases, if the wet-bulb reads higher than the dry-bulb, the observation should not be recorded, the instrument being for the time not in proper working order. If the water on the muslin is frozen, the readings are quite good, since evaporation takes place from ice as well as from water. But if the muslin be dry, it must first be wetted and then allowed time to freeze before the observation is taken. A useful rule to observe in frosty weather is, to immerse the bulb and conducting-thread in water after every observation, by which sufficient ice will still be adhering to the muslin at the time of the next observation. When the temperature of the air rises above 32°, the wet-bulb must be plunged into warm water to melt any ice that may remain on it, after which it must be allowed sufficient time to cool before being observed.
327. To keep this instrument in working order one or two things require special attention. Both thermometers must be exactly the same; for if one is filled with mercury and the other with spirit, or if they contain different quantities of the same fluid, the readings will in either case be vitiated. All starch or foreign matter should be washed out of the muslin and cotton. The water used should be pure ; for if lime be dissolved in it, the muslin will soon be coated with a calcareous incrustation. Rain or distilled water should be used. The muslin ought to be changed when covered with dust or other impurities, and care should be taken not to touch the muslin with the fingers, otherwise it will get slightly greased, and capillary attraction be thereby interfered with. The bulbs of the two thermometers should be made to project 1} or 2 inches below the scales. The thermometers should also be a little apart from each other, and the glass vessel be, as in the figure, as far removed as possible from the dry-bulb.