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 formulæ (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:—
| Temperature by the Dry Bulb Thermometer. | Difference between Dry-bulb and Wet-bulb Readings. | |||||
| 2° | 4° | 6° | 8° | 10° | 12° | |
| Degree of Humidity. | ||||||
| 34° | 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°. 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°, occasionally.