224 
Relation of Moisture in Air 
l April, 
as io per cent, or ij grains to the cubic foot. The resulting 
figures which the latent heat demanded for the evaporation 
of Lhese ij grains of moisture into the air supposed is 74^-°, 
and 82 per cent of humidity. How far this condition of the 
air may be more comfortable than the original one of 85° 
and 50 per cent of humidity is questionable ; but it is 
apparent that the limit of possible cooling of air of 85°, by 
evaporation of moisture, is found when its humidity is not 
to exceed 50 per cent of saturation. A similar computation 
applied to air of 105° (in which air there is a little more 
moisture than in that supposed at 85°) gives, for the addition 
of 15- grains of moisture to the cubic foot, air of 94I- 0 tem- 
perature and 48 per cent of humidity — -an atmosphere which 
may be, in some degree, more comfortable to a person at 
rest than the normal one. It is evident that, for efficiency 
in cooling air of 105° by evaporation from moistened sur- 
faces, the humidity of the air to be cooled should be less 
than 30 per cent. As to ultimate healthfulness of the 
moistened air, it seems to be unquestionable that the supply 
of moisture ought to promote disease. 
We have, however, in each year in our country, a few 
days or parts of days (perhaps, in the Southern Middle 
States, ten to twenty days in different years) when the range 
of thermometer and the dew-point make it feasible to adopt 
this means of reducing the apparent heat of the air. The 
attempt has been frequently made, with provoking failure 
to the projectors. Its success depends upon not only the 
exaCt condition of relative humidity and temperature, but 
also on the proportion of surface of evaporation to the 
quantity of air to be supplied. The Indian ratio is one or 
two persons to 6 to 8 square yards of wet surface. But the 
most provoking cause of failure has been, that while there 
are ten to twenty hot dry days in any year, there are also 
twenty to thirty hot damp ones, and the application of the 
cooling apparel to the hot air on these days has produced 
such an effeCt of sultriness that the whole has met with 
instant condemnation. 
The last relation of moisture to air to be considered at 
this time is that which proceeds from the effort to cool air 
artificially. The fallacies of the attempts to effeCt this 
purpose can be made very apparent. Even the smallest 
decrement of heat is obtained only at great cost. The 
quantity of cooling of air in summer is, to be sure, only 
about one-fourth that of heating in winter. Taking the 
ideal temperature of 70°, there may be 15 0 to come off in 
summer as generally as 6o° to be added in winter ; and sup- 
