32 



9 oz. per 1,000 cu. ft. — the "absolute humidity;" (2) by comparing the 

 "absolute" with saturation, e.g., 9 /i 8 or ^0 per cent. — the "relative 

 humidity." Of these the latter is the more useful. In the economy of 

 nature evaporation plays a very important part. If evaporation from the 

 ground is too rapid, the soil becomes parched and unfit for sustaining the 

 plants growing upon it; if evaporation from the plants is too rapid, they 

 wilt; if evaporation from our bodies is too rapid, we are conscious of 

 feverish distress, while on the other hand if it is too slow, the air is 

 oppressive and the perspiration, instead of evaporating, stands out in 

 beads. These various phenomena are controlled by the relative humidity, 

 not by the absolute. If the air has a low relative humidity the evapora- 

 tion will be fast, but if a high relative humidity, it will be slow. Hence 

 the "relative" humidity at any time furnishes us with much more valu- 

 able information than the "absolute." In general practice the word 

 "humidity" is used alone to stand for "relative humidity," and will fre- 

 quently be so used in the following pages. 



The Wet- and Dry-Bulb Thermometers. 



But the absolute method of determining the relative humidity is very 

 laborious and very exacting — one dare not even breathe on the U tubes, 

 for the moisture that would condense on them from the breath would spoil 

 the determination entirely in many cases (a fact which we learned by bitter 

 experience), and it could only be employed where delicate balances were 

 available ; hence if humidity determinations were to have any extended 

 application, some simpler method had to be evolved. 



Now evaporation has a cooling effect, as any one may prove by the 

 aid of two thermometers which read the same when dry. Wet the bulb 

 of one with water as warm as the room and hold them side by side. In a 

 very few moments the wet one will read several degrees lower than the dry 

 one. This is explained by the fact that heat is used up in turning water 

 into vapor, a familiar illustration of which is to be found in the kettle 

 heating on the stove. The water becomes warmer and warmer until at 

 last it begins to boil. Despite the fact that heat still passes into it the 

 temperature remains at boiling point; the heat is absorbed in turning the 

 water into vapor. The heat thus used it called latent heat, because it 

 produces no change of temperature. It takes 5.38 times as much heat to 

 vaporize the water as to heat it from freezing to boiling. Now. whenever 

 vaporization of water takes place this same latent heat is absorbed. If 

 there is no fire to provide it then it must come from the evaporating 

 water, the air, and surrounding objects. At first, the evaporating water 

 on the wet thermometer draws most of its latent heat from the thermo- 

 meter itself, hence the temperature is lowered. The faster the evaporation 

 the greater amount of latent heat required in a given time, and hence the 

 greater the reduction in temperature. But the rapidity of evaporation is 

 controlled by the relative humidity of the air ; the lower the humidity the 



