METEOROLOGY. 209 
winds ; on an average the air is further removed from the condition of 
saturation during north-east winds, than during southern and south-west 
winds, although occasionally decided exceptions to these general principles 
occur. 
As the formation of vapor depends partly upon the temperature and 
partly upon the presence of water, it follows that the dryness of the air 
must increase with the distance from the equator and from the sea, an 
inference well established by observation. 
A partial condensation of the vapor of the air takes place whenever it is 
brought into contact with a sufficiently cold body, whose temperature is lower 
than that which corresponds to the density of the existing vapor. A part 
of the latter is thus deposited on the cold body in the form of minute 
vesicles. The moister the air the less the temperature of the body needs to 
differ from that of the air. In this way we explain the coatings of moisture 
on the window panes of an inhabited room, whose temperature is higher 
than that of the external air, so that the panes are coated from within, a 
phenomenon which is only exceptionally reversed, and this especially in 
spring. Pl. 23, fig. 55, shows the symmetrical manner in which these 
vesicles are deposited. 
Of Dew. 
The local deposit of moisture referred to well illustrates the theory of dew 
in general. When after sunset the sky is clear and the air calm, a great 
radiation of heat takes place from the earth, accompanied of course by a 
diminution of temperature. The layers of air in contact with the earth and 
objects upon it become cooled, and when this reduction of temperature 
reaches the dew point, a deposit of vapor ensues. 
To determine the amount of dew we make use of a drosometer, an instru- 
ment still very imperfect and inadequate. By drosometer is to be understood 
a balance, the shorter arm of which carries a plate, readily bedewed, the 
other arm a counter weight. Instead of the plate, as used by Wilson and 
Flaugergues, we may use to advantage a bunch of wool or eider-down, to be 
appended to the shorter arm. Wells, to whom we are indebted for the first 
rational theory of dew, used flocks of wool weighing ten grains, pulling 
them into a mass of about two inches diameter, and measuring their increase 
in weight after exposure. Lambert proposed to expose well washed and 
curly hair to the air, and then to ascertain its increase of weight in a given 
time. 
The following generalities respecting dew are furnished by Wells. Dew is 
formed in greater quantity during clear calm nights, especially when clear and 
calm weather succeeds to stormy. Clouds prevent the formation of dew, as 
also brisk winds, which constantly replace the cooled air by warmer. Little 
dew falls during a night when the sky is cloudy and the wind still ; none at 
all when a cloudy sky and windy weather are conjoined. Objects interposed 
between the radiating surface and the clear sky, produce the same effect as 
383 
