213 Relation of Moisture in Air [April, 
another few hours. Under this change heavily-frosted win- 
dows become the rule, and they indicate a dew-point above 
the temperature of the panes of glass, at once. So dry and 
arid are these winds, however, that with the continuance of 
a north-west wind for a single day all traces of window- 
frosting will have disappeared. 
This test of the dew-point by the window is very accurate. 
As an instance I will quote that on one day in this present 
month, in Philadelphia, with the thermometer in-doors at 
64°, without fire, a fall of temperature to 54 0 outside, pro- 
duced immediate condensation on windows, showing 75 0 to 
8o° of humidity. Now if a difference of only io° produces 
this indication for humid air, the want of such an indication 
with difference of up to 65° must manifestly be a very dry 
air. By aCtual trial in well-warmed and ventilated rooms, 
the writer has found the dew-point far below the freezing- 
point of water, in rooms where the sensation of dryness, 
which is held to accompany the heat of the furnace when 
not supplied with water for evaporation, certainly did not 
exist. 
It is proper, when alluding to the dampness of houses, to 
advert to one of the most striking differences between 
England and Western Europe and the northern United 
States, in the necessities which climate imposes in the rela- 
tion of humidity in air to the health, in this particular 
regard. We find all foreign authors speaking in unequivocal 
language of the great danger of inhabiting a newly-built 
and consequently imperfectly dried house. One English 
writer, whose name I cannot recall, but I remember to have 
been of much eminence, asserted that no house ought to be 
occupied until a year after completion. Many writers on 
ventilation join in estimating by figures the quantity of 
moisture in the new walls, and demonstrate the dangers of a 
residence where the excess of moisture in the air, the want 
of permeability of the walls, and the increase of conductivity 
of heat through the damp walls, will have produced such a 
tomb-like house. While in America we are fully alive to 
the danger in the house, from air overladen with moisture, 
as from a damp cellar or location in a damp place or vicinity, 
and appreciate that rheumatism and consumption, with 
scarlet fever for the children in winter, when the house is 
thoroughly warmed, and typhus for adults in mild weather, 
are the possible, if not probable, penalties for living in such 
a house ; yet the new house, from its inherent dampness, 
only is considered at least as but objectionable in a small 
degree. Except that dampness exists from other sources 
