i 8 yS'] to Health and Comfort . 223 
incapable of producing a cooling effedt, while as the tem- 
perature or the dew-point should fall the current would 
become a pleasant one. With a high temperature and dry 
air the cooling effedt of a current of air (even at ioo°) may 
be very pleasant in the sensation, but will be attended with 
sun-burning (even without exposure to the sun), and blisters 
will be produced by the excessive deprivation of moisture 
from the cuticle or surface of the skin. With 8o° of tem- 
perature and a high dew-point a strong breeze is not 
unpleasant, nor likely to be injurious after the person shall 
have acquired some accustomed habit of body to endure it ; 
but at 70° and a low dew-point, which is the only possible 
condition of heated air in mid-winter, the annoyance of a 
current of even 5 feet per second and its unhealthiness are 
positive fadts. 
§. There remain to be considered two more relations of 
moisture in air to health and comfort. First, the effedt of 
evaporation of water by the air itself in summer, in reducing 
the temperature to one of comfort ; and secondly, the effedt 
on the moisture condition of the air of summer, when it is 
attempted to cool air by artificial means. 
The cooling of air, by spontaneous evaporation from ex- 
tended surfaces, is of frequent pradtice in hot countries 
by the wealthy inhabitants near the banks of rivers where 
the water-supply is abundant. The condition of the air 
which makes it practicable is one of great heat, and of a 
relatively low dew-point ; and the summers of Egypt and 
of parts of India, especially of Bengal, give opportunity to 
employ this method of cooling air. If it is accepted that 
the temperature of the air in the shade, in the localities 
referred to, will sometimes run from 85° to 105° for many 
consecutive hours, accompanied with, say, 50 per cent of 
moisture for the 85° of temperature, or, say, 30 per cent for 
the 105°, then the evaporation of moisture from wet surface 
can be relied upon to produce a comparatively comfortable 
atmosphere. Air at 85°, with 50 per cent of moisture, has 
quite exadtly the same quantity of moisture, per given 
volume of air, as that at 7 o° and 70 per cent of moisture. 
So that if it could be cooled without adding moisture at all 
it would then reach the point of comfort for the clothed 
inhabitant of the temperate zone. If the dew-point of 85° 
is brought up to 80 per cent, or above, the air becomes 
intolerably sultry, and at go per cent quite suffocating ; so 
that the greatest addition of moisture practicable to the 
supposed air of 85° and 50 per cent dew-point may be taken 
