THE WEATHER. 223 



the atmosphere from the sea, are wafted in every direction 

 over the surface of the earth. This distribution goes 

 constantly forward, and the air, however transparent and 

 apparently free from moisture, is indeed always loaded 

 with these invisible vapors. This is evident from the 

 following experiment. 



In a summer's day place an empty tumbler upon the 

 table, and it remains hour after hour, dry. Pour now 

 cold water into it until it is half filled, and the moisture, 

 condensed by the cold, will stand in dew drops upon the 

 outside of the glass. The same effect will be produced 

 if any other cold substance is introduced into warm air. 

 There are two ways by which the vapors, thus dif- 

 fused throughout the air, are made to fall. These we 

 shall describe. 



3. CONDENSATION. 



(a) Condensation by Cold. 



Such is the constitution of our atmosphere, that it will 

 receive into itself much more moisture when warm than 

 when cold. The consequence of this is that if warm air, 

 previously loaded with moisture, becomes, from any cause, 

 cold, it can no longer retain that moisture in transparent 

 solution. It consequently assumes again the form of 

 water, small, very small drops of water, too minute to 

 be seen singly by the naked eye, but forming altogether 

 a hazy appearance, which we call fog, or mist, or cloud, 

 according to circumstances. There are many ways in 

 which this principle is illustrated. 



The breath which proceeds from our lungs is always 

 loaded with moisture. It is nevertheless invisible unless 

 in a cold day, when it appears in the form of a cloud 

 issuing from the mouth. This apparent cloud does not, 

 however, appear to commence precisely at the lips. The 

 warm air must proceed a little distance before it is so 

 mixed with the cold air around as to have the effect 

 produced. 



When in cold weather, green wood is burnt for fuel, 

 a cloud of visible vapor issues from the top of the 

 chimney. That is, the vapor, which was invisible as it 

 comes from the fire, and as it passes up the flue of the 



