METEOROLOGY. 



90, there would be the means of supporting an 

 additional quantity of steam, and evaporation 

 would again go on, until as much were distributed 

 through the air as of itself would counterbalance 

 1-36 inches of mercury, or about -^d of the weight 

 of the whole atmosphere. With every new ad- 

 dition of heat, new evaporation would go on, 

 which would be of the silent kind up to 212, 

 when the full pressure of the atmosphere would 

 be reached. But as the water on the earth's 

 surface does not rise much above 80 even at the 

 equator, and is far below this temperature in 

 other regions, the whole weight of vapour in the 

 air rarely amounts to one inch of mercury; so 

 that, if the pressure due to nitrogen and oxygen 

 by themselves were 29^ inches, the whole pres- 

 sure, including the vapour, would be only about 30 

 inches, and at the utmost 30^ inches. 



It has been stated, that at 80 vapour rises till 

 as much exists in the air as weighs an inch of 

 mercury ; and that if the heat be increased, an 

 additional quantity can be produced and sup- 

 ported. Let us now consider what must happen 

 if the air were saturated with all that could be 

 maintained at 80, and if the temperature were 

 then lowered, say to 60. While at 80, the 

 vapour may amount to an inch ; at 60, it 

 amounts only to '52, or little more than half an 

 inch. If, therefore, the full quantity which can 

 subsist at the higher temperature has been pro- 

 duced, and if the temperature then descend to the 

 lower point, there will be nearly half an inch too 

 much for the reduced temperature, and this excess 

 will be thrown out in the state of visible non- 

 elastic vapour, or fall down as liquid drops. There 

 will be either some kind of fog, cloud, or mist, or 

 the aggregation of these into watery coherence. 



Tables have been formed, the result of accurate 

 experiment, shewing the elasticity of the vapour 

 that can be maintained at each degree of Fahren- 

 heit, from o to 90, expressed in inches of baro- 

 metric pressure. 



If less steam is formed than could be sup- 

 ported at the temperature, there is said to be a 

 certain amount of dryness or thirstiness in the air; 

 meaning that there is room for further evaporation. 



If the quantity of vapour in the air is less than 

 what is supportable at the temperature for the 

 time being, there is some lower temperature which 

 this quantity would completely saturate. Such 

 temperature is called the temperature of the dew- 

 point. The dew-point is the lowest point to 

 which the air can be cooled down without giving 

 out visible moisture. If the air were saturated, 

 the temperature and the dew-point would be 

 the same ; if the air is dry, or not saturated, 

 the dew-point temperature is below the air tem- 

 perature ; and the difference between the dew- 

 point and the temperature of the air is a measure 

 of the dryness. 



The number of degrees of Fahrenheit between 

 the air temperature and the dew-point temperature 

 is not, however, the exact estimate of the dryness, 

 as will be seen from the following example. Sup- 

 pose the temperature of the air 80, and the dew- 

 point 70, then the amount of additional vapour 

 that could be supported would be found thus : 



Vapour sustainable at 80, I-ooi inches. 

 70, -727 inch. 



Difference, . . -274, above I -4th of inch. 



Thus a difference of 10 of temperature makes a 

 vacancy for a fourth of an inch of vapour. Take 

 now a difference of 10 farther down in the scale. 

 Suppose the air 40, and the dew-point 30 : 



Vapour sustainable at 40, -264 inch. 

 * . 30, -186 



Difference, . . 078, about l-i2th of. inch. 

 So that a difference of 10 between 70 and 80 

 makes a dryness or deficiency three times as great 

 as a difference of 10 between 30 and 40. Water 

 would disappear three times as fast in the one 

 case as in the other. 



As the mercurial column expresses only the pres- 

 sure of the sum-total of the atmosphere, some other 

 means must be adopted for finding the amount 

 of vapour by itself. Instruments used for this end 

 are called hygrometers ; from the Greek hygros^ 

 moist, and metron, a measure. 



Apart from instruments, we judge of the dryness 

 of the air, or of the additional amount of vapour 

 which it could sustain, by the time required to dry 

 wet bodies, such as the ground, or wet clothes 

 hung in the air. 



The chief instruments for determining with 

 accuracy the dryness of the air, and the actual 

 amount of aqueous vapour that it contains 

 technically its hygrometric condition are Daniell's 

 hygrometer, and the wet-and-dry-bulb thermom- 

 eters. 



The principle of Daniell's hygrometer is to cool 

 the air down upon some surface till dew appear, 

 and to mark the temperature when this happens. 

 Thus, suppose a tumbler standing in the open air 

 is cooled by pouring in cold water, or any other 

 cold liquid, until the sides of the tumbler are 

 covered with dew, the temperature of the glass at 

 the moment that the dewing begins will be the 

 temperature of the dew-point. In the actual 

 instrument, the cooling is produced by the rapid 

 evaporation of ether. But as the process of 

 observation is rather delicate, and attended with 

 trouble and expense, the wet-and-dry-bulb ther- 

 mometers have come into more general use. 



The determination of the dew-point by the wet- 

 and-dry-bulb thermometers depends on the effect 

 of evaporation in causing cold. When water 

 passes into invisible vapour or steam, it absorbs 

 from whatever substances it touches a large 

 amount of heat, and the more intense the evapora- 

 tion, the greater will the cooling be. In applying 

 this principle to measure the humidity con- 

 tained in the air, two thermometers are taken 

 one of the ordinary construction, which serves 

 simply to give the temperature of the air ; and 

 the other having its bulb covered with a piece of 

 rag, which is kept constantly wet by communi- 

 cating with a cup of water by an absorbent wick. 

 The water round the bulb will be constantly 

 evaporating when there is any dryness in the air ; 

 and the greater the dryness, the more intense the 

 evaporation, and consequently the greater the cool- 

 ing of the bulb. Hence this moist-bulb ther- 

 mometer will fall in proportion to the dryness of 

 the air at the time. We have therefore only to 

 compare the two thermometers the one giving 

 the air's temperature, and the other a reduced 

 temperature depending on the rate of evapora- 

 tion or the degree of thirstiness in order to 

 judge of the comparative quantity of moisture ex- 

 isting in the surrounding atmosphere. Tables 



