224 THE WEATHER. 



chimney, because the air which carried if. teas warm, 

 becomes visible as soon as it is exposed to the cold air 

 above. It will be observed in this case as in the other, 

 that the condensation does not take place until the vapor 

 has ascended a foot or two above the top of the chimney. 

 A short time since a large congregation were assembled 

 in a church in Boston at an evening lecture. The heat 

 and moisture produced within the house by the crowd, 

 began to be very great ; and when, towards the close of 

 the evening, tho door was opened, the cold air rushed in 

 and produced such a condensation of moisture in the air 

 that it was mistaken for smoke and produced a momen- 

 tary alarm of fire. 



(6) Condensation by the Rarefying of the Air. 



It is observed that generally when from any cause the 

 air becomes rarefied, there is a tendency to condensation 

 of the moisture which it contains. It has been previously 

 shown that when the air becomes rarefied by diminution 

 of the pressure upon it, (see page 114) its temperature is 

 diminished. Now perhaps the condensation of the 

 vapor is owing to this cooling of the atmosphere, though 

 it is on the whole probable that the diminution of density 

 has a direct influence. 



A proper consideration of this principle will explain 

 the fact at which so much surprise is sometimes expressed, 

 namely, that storms move against the wind. That is, 

 a northeast storm begins in New Orleans and works its 

 way against the ivind to Boston. This, however, instead 

 of being surprising, is precisely what, with a little reflec- 

 tion, we should have been accustomed to expect. The air 

 over Mexico is rarefied by the sun, and rises. The 

 atmosphere from Louisiana rushes in to supply the 

 vacancy. Virginia then and the states around it must 

 send on a supply to Louisiana. New York must 

 part with a portion of her atmosphere to fill the void in 

 Virginia ; and a breeze from New England to New York 

 closes the process. Now it will be evident that the rare- 

 faction which commenced at the southwest gradually 

 will extend towards the northeast, and as it advances, 

 will produce condensation, that is, rain. But the wind at 



