1878.] 
to Health and Comfort. 
203 
to attain the desired purity is from 40 to 60 cubic feet of air 
for each person each minute, where the contents of the 
rooms can be considered as furnishing a portion of the 
supply when occupancy is only for a part of the time. 
Much of the air may not be supplied through the heating 
apparatus. In cold weather, when the levity of the heated 
air within a building, compared with the colder air on the 
outside, produces a great pressure of outer air near the 
ground, the leakage of air at cracks of the door and 
window-frames, at the top of the building, and its replace- 
ment by colder air through similar apertures at the bottom, 
furnish a much larger volume of air than is generally sup- 
posed. The strong winds also seek such leaks. Some 
permeability of walls, even of boards well painted, is avail- 
able for the diffusion of vapour and of gases in a measure. 
So that the proper quantity of air to be supplied by an 
apparatus becomes a question to be considered, in each in- 
stance, on its own merits. But the fadt still remains, that 
for each adult or child in health, 40 to 60 cubic feet of fresh 
air must be estimated as provided, either by arrangement or 
surreptitiously, for each minute they may occupy a room or 
place, although not necessarily during each minute of the 
day and night. This fresh air must be derived from out of 
doors. 
Accept, for the sake of argument, the average temperature 
and dew-point of Philadelphia, in January, February, and 
March, of 1844, as reported in Prof. Bache’s meteorological 
and magnetic observations. The mean temperature of 
those three months was 34 0 , with an average dew-point of 
25 0 ; barometer 30 inches, from hourly observations, giving 
68*8 per cent of saturation. Using Guyot’s Psychrometrical 
Tables, Regnault’s data, 1*57 grains of aqueous vapour 
exist in a cubic foot of such air. These, in Philadelphia, at 
this season, are the unquestioned properties of the air from 
which is to be furnished the fresh air of ventilation. If 
heated to 70° without increment of moisture, the dew-point 
remains unchanged, and the same 1*57 grains of moisture 
appertain to the enlarged volume of air, now increased 
8*2 per cent by expansion. The hygrometric condition of 
this air is but 1*44 grains per cubic foot, or but 18 per cent 
of saturation. The summer hygrometric condition of air 
can be derived from the same source. The three months of 
July, August, and September give 71 0 average temperature, 
with 6o° dew-point, or 68*3 per cent of saturation. Suppose 
we take the 68*3 per cent, and consider it the proper condi- 
tion for the air of ventilation at 70° ; it then follows that 
