210 
Relation of Moisture in Air 
[April, 
change of temperature or relative moisture of the air, are of 
necessity confined to the cold, or at least cool, season, with 
infrequent attempts to obtain an artificial degree of cold in 
extreme hot weather. In moderate weather the vicissitudes 
of temperature, and of humid condition of the air, are en- 
dured with the expression of discomfort and the tacit 
admission, on all hands, that our great day-by-day varia- 
tions of the mild season are harmful to the feeble or sickly. 
But these daily changes of temperature and of hygrometric 
condition are of small account with those which in the 
Northern States accompany the season of cold. The change 
of climate from that which accompanies, during any of the 
winter months, our warm south winds, to that which accom- 
panies a great north-western wind-wave, which may follow 
the southerly breeze within twelve hours, — a change from 
50° to 6o°, with 80 per cent saturation, to even below zero, 
with an unascertainable dew-point, — such a change is trying 
in the extreme. The prevalent disease of the land is con- 
sumption of the lungs, and these changes are disastrous to 
those who are suffering from this complaint, and to the 
healthy these changes are held to be fraught with danger. 
§. A very simple and commonplace observation will make 
the general condition of air in rooms in winter, as regards 
humidity, the subject of positive demonstration. During 
the season of winter in our climate, after a continued spell 
of cold weather, the exhibition of condensed moisture or of 
frost on the window-panes is very infrequent. The usual 
provision of glass in windows throughout the northern 
United States is in single thickness, not double plates ; the 
latter arrangement being decidedly exceptional as a means 
of preventing transfer and loss of heat at the windows. 
The temperature of a pane of glass which is interposed 
between two temperatures of still air — that is, of air devoid 
of currents, except those generated by the differences of 
temperatures of the air on either side and the glass — is ob- 
viously that of a mean between the said two temperatures 
of air. With so good a conductor as glass, and with plates 
as thin as ordinary window-glass, the conductivity of the 
glass may be assumed to be perfect, and both sides of a 
pane can be deemed to have the same mean temperature. 
But the temperature of the air on each side of a pane 
differs from either that of the external air on the one side, 
or of the room on the other. On the outside, the layer of 
cold air in contaCt with the pane ascends slowly as it is 
heated, and the vacuity which is formed at the bottom of a 
