222 
Relation of Moisture in Air 
[April, 
marked injury to the occupants or to visitors. The quantity 
itself seems to be almost constant for all temperatures or 
hygromations of the air, and to be a slight addition only to 
the moisture in the normal air out of doors at any time. 
§. The effedft of draughts or currents of air upon any person 
exposed to them is materially modified by the hygrometric 
condition. In still air in winter the comfortable tempera- 
ture of a room in general hygrometric condition has been 
stated at 70°, but a current of air upon the person at this 
temperature is uncomfortably cold from the rapid abstrac- 
tion of heat, while in summer the same temperature, accom- 
panied by the summer dew-point, will be warm enough to 
demand light clothing, and a current of the same velocity 
will not be objectionable; in other words, draughts which 
cannot be tolerated in our houses in winter become pleasant 
breezes in summer. Not only the speed or rate of velocity 
of the current of air is to be taken into account, but its 
avidity to take up moisture from the skin as indicated by 
its dew-point. So long as the hygrometric condition of the 
air is such that will absorb moisture below 98°, a blast of it 
at some rate of current will be a cool one. 
One fans himself in our climate, when the thermometer 
stands at ioo°, with a sensation of relief. This feeling of 
cold, from air of high temperature, when in motion, pro- 
ceeds from the rapid removal of the stratum of warm and 
nearly saturated air in contact with the person, and its re- 
placement by fresh air, which is not only cooler, but 
which has not yet become saturated or charged with mois- 
ture by contact with a moist surface like that of the skin. 
In no one of the changes in the three forms of matter — 
solid, liquid, and gaseous — is there so much heat taken up 
as in the change from a liquid to a gaseous (or vaporous) 
form, and in no other body or substance is so much heat 
absorbed or become latent as in the formation of steam from 
water, or, in other words, in the process of evaporation ; 
and the quantity of heat taken up by the moisture which a 
dry air abstracts from the skin is so great that the mere dif- 
ferences of temperature of the air from that of the skin may 
almost be neglected in the statement ; and it is very nearly 
correct to assert that the cool sensation from a breeze in 
summer proceeds entirely from the evaporation of moisture 
thereby induced. 
Upon this basis it may be noticed that a current of satu- 
rated air at ioo° would neither remove heat by its contact 
nor by induced evaporation, and consequently would be 
