== ~ 
“ Saat a: 
243 
perfectly dry as well as transparent, and exerting no influence 
upon our feelings. 
The relative humidity, therefore, must not be confounded with 
the absolute amount of vapour present in the atmosphere, or—as it 
has been sometimes called—the absolute humidity, which varies 
very differently from the former, and is often least when the 
‘relative humidity is greatest, and vice versa. The absolute amount 
of vapour depends upon temperature, the air having an increazead 
capacity for aqueous vapour as the temperature increases. Hence 
in summer the amount is much larger than in winter, whereas in 
the case of the relative humidity it is just the reverse. Here the 
maximum is in winter. For though the sun at this season, from 
its reduced evaporating power, raises but little vapour from the 
earth’s surface, the air nevertheless is extremely humid from 
increased condensation due to cold. 
In proportion to the temperature also will be the elasticity of 
the atmospheric vapour, or its power to sustain more or less of 
the mercurial column. This elastic force is given for each month 
in the third column of the above Table, and to a certain extent 
the absolute amount of vapour is represented by it. And it will 
be seen, agreeably to what is above stated, how this quantity 
increases as the year advances, attaining its maximum in the sum- 
mer months of July and August ; it then gradually declines till it 
gets to a minimum in December and January. 
The relative humidity is partly indicated in the second column 
of the same Table by the mean depression of the dew-point below 
the temperature of the air, this depression increasing with the 
dryness of the atmosphere, and therefore usually greatest in 
spring and summer, though varying from month to month 
according to the weather. The relative humidity, however, is 
more exactly given in the fourth column, where “ saturation is 
assumed as 100, and perfectly dry air as zero.” The higher the 
figures, therefore, in that column, the more nearly the humidity 
of the air approaches to complete saturation. 
