Glaisher.] DIFFERENCE OF WET AND DRY BULB. 31 vicinity. The water-vessel should always be supplied with rain or distilled water. 8. In frosty weather the water in the reservoir will be frozen, but this is no reason for the suspension of the observations; if the water upon the muslin is frozen at the same time, the readings are perfectly available. If the muslin be dry, it is necessary that it be wetted a sufficient time before the observation for the water to become frozen. When the weather is frosty, the muslin should be immersed in water after every observation. Before use, the cotton lamp-wick should be washed in a solution of carbonate of soda, and pressed whilst under water throughout its length. In use it should be of such extent that the water conveyed be sufficient in quantity to keep the muslin moist, without having a drop attached to it. The amount of water supplied can be increased or diminished by increasing or diminishing the extent of the conducting-thread. The mean monthly readings of the dry-bulb thermometer, corrected for diurnal range, are inserted in Table 5 in the Appendix (p. 446). 9. The mean monthly readings of the wet-bulb thermometer, similarly corrected, are contained in Table 6 in the Appendix (p. 447). 10. From the readings of the dry and wet-bulb thermometers, the dew-point is calculated by multiplying the difference between the readings of the dry thermometer and that of the wet by my factors in the following Table, and subtracting the product from the reading of the dry thermometer. 11. Temperature of the dew-point in summer is lowest a little before sunrise; as the sun ascends, evaporation increases, and the air receives a greater quantity of vapour, consequently the temperature of the dew-point increases till about noon, when it attains its maximum, and remains at this value till after the temperature of the air begins to decline, and then gradually decreases till the following morning. Table, showing Glaisher's factors, by which the differences of readings of the dry-bulb and wet-bulb thermometers are to be multiplied, in order to deduce the difference between the temperatures of the air and dew-point. Glaisher.] DIFFERENCE OF WET AND DRY BULB. 33 In winter its minimum occurs some hours before sunrise, and its maximum occurs at about the time of the maximum temperature of the air, and then gradually declines to the following morning. 12. The use of this Table is very simple. Example.In Table 5, June 1841 the mean temperature of the air was 56.4, and in Table 6, that of evaporation for the same month was 52.6. Required the temperature of the dew-point? In Table 7, the dew-point observed was 49°.1. 13. Whenever the air of the atmosphere is saturated with moisture, the temperature of the air, evaporation, and dew-point is the same. Whenever the temperature of the air is above the temperature at which it is saturated by the moisture which it contains, a portion of all exposed water undergoes a gradual diminution of bulk, and ice imperceptibly wastes away by rising in the atmosphere. Whenever air is cooled below the temperature at which it is saturated, a portion of moisture is separated, in proportion to its newly-acquired temperature. The 14. The air of the atmosphere is, however, generally in that state, that a depression of some degrees is necessary for the deposition of a portion of the water which it contains, and it will therefore take up water. further it is removed from saturation, the more rapidly evaporation proceeds, and as it proceeds, the heat absorbed in the conversion of the thin film of water around the wet bulb into vapour will cause the reading of the wet to decrease below that of the dry thermometer, and this depression in hot weather in this country may amount to 20°; in India and Australia at times to 30°. In cold weather, the difference will in general be small, but evaporation will proceed when the temperature is below 32°: great care is required in the observation when the temperature of the air is somewhat below 32°. At times it will happen, when the temperature has descended below this point, that the water on the bulb has not begun to freeze, and the reading of the wet bulb will be the higher of the two: these readings must not be recorded; when water has begun to freeze, the lower readings are taken. When the temperature is below 32°, the reservoir of water is useless, and the bulb must be moistened, some time before the observation, by a sponge or brush. When the temperature of the air has risen above 32°, the bulb of the wet thermometer, with its conductingthread, should be immersed in warm water, for the purpose of removing any ice that may still remain on the bulb. 15. Having determined the temperature of the dewpoint, the elastic force of vapour is at once extracted from a properly-constructed Table. Table 8 in the Appendix has been thus formed. 16. Position of the Thermometers. They should be placed in a position sheltered from the direct rays of the sun; at such a distance from walls as not to be influenced by reflected heat; protected from rain, and from the effects of radiation, with the bulbs freely exposed to unimpeded circulation of air from all sides. 17. Stand for carrying the Thermometers.-In fig. 13 is an accurate representation of such a stand: it consists of a frame of deal boards, through all parts of which the air passes freely, for the purpose of carrying away any heat which may be conducted through them. It can be constructed by any carpenter. The whole revolves; its inclined side is always turned towards the sun. On its face, the dry and wet bulb, the maximum and the minimum thermometers are Fig. 13. attached in such way that their bulbs project two or three inches below the frame, and about four feet above the ground. LESSON IV. VAPOURS. "For what is your life? It is even a vapour, that appeareth for a little time, and then vanisheth away." 1. Weight of Vapour in a Cubic Foot of Air.—A cubic foot of vapour at 212° and under a pressure of 30 inches, weighs 258-4 grains; and air expands 1 491.13, or 0.0020361 for every increase of 1° of heat; it there |