202 
Relation of Moisture in Air 
[April, 
deaths in the Black Hole of Calcutta were the result of 
excess of moisture, rather than of heat or want of air 
per se. There are travellers’ tales of regions on the Red 
Sea, and near the mouth of the Persian Gulf, where men 
cannot breathe in summer for the heat combined with 
moisture. 
In the same way that the effedt of a nearly saturated 
atmosphere has been examined, that of a very dry one may 
be investigated. To the sense of feeling all air may be said 
to be dry below 35°. The small amount of vapour present, 
and possible to exist as vapour below this point, reduces the 
conductivity so that the chilliness, to a great degree, disap- 
pears, even in a saturated air. Yet even here the cold- 
producing effedt of a high dew-point is felt in a wind, so 
that from 15 0 to 35 0 the N.B. wind of our Eastern States is 
a very raw one. But, on the other hand, with a dry air 
from 40° below zero to the freezing-point, the immediate 
sensation of cold by the aCtive man, well clad with porous 
clothing, is yet endurable, and with habit becomes almost 
pleasant. Still these temperatures are not those suited to 
civilised life, either physical or mental. As has been before 
noticed, we have in the Northern States about live months 
of the year when the temperature ranges from o° to 50°, and 
consequently when our civilised avocations demand artificial 
heating. The winter climate of the Eastern, Northern, and 
Middle States is one of great vicissitudes, with extremes, 
both of temperature and of hygrometric conditions, following 
each other rapidly. In the North-Western States it seems 
that a somewhat greater uniformity of temperature and a 
much more uniform hygrometry exist during the winter 
months, but in the Middle Western States the irregularities 
appear to be as frequent as in the Eastern States. Except 
that the length of the winter season is a little cut short, and 
the excessive cold is a little alleviated in the southern por- 
tion of the belt of country I have designated, much the 
same phenomena of climate exist all through the States 
north of the 40th parallel of latitude. Throughout this 
territory it has become recognised that the minimum tem- 
perature of comfort for heated and ventilated rooms can be 
stated at 70°, with an admitted — and generally supposed in- 
explicable, if not unreasonable — demand for 75 0 to 78° in 
some localities and at some times. 
§. Ventilation means a supply of fresh air to the occu- 
pants of a house, workshop, or meeting-room of any kind ; 
and, as a final result, the quantity of such supply needed 
