70 



PHYSICAL GEOGRAPHY OF THE SEA. 



tatory movement is. also variable ; its power is often very great, 

 but sometimes water-spouts pass over small vessels without in- 



juring them. They are more frequent near the coast than on 

 the high seas ; and are more commonly seen in warm climates. 

 They seem to occur particularly in regions where calms frequently 

 alternate with storms, which is not to be wondered at, since they 

 owe their origin to miniature storms or whirlwinds. 



How do the aqueous vapours with which evaporation impreg- 

 nates the atmosphere, again descend upon the surface of the 

 earth ? 



Everybody knows that when in summer a bottle filled with 

 cold water is brought into the room, it soon gets covered with 

 thick dew-drops, which presently trickle down its sides, although 

 it was perfectly dry on entering. Whence does this moisture 

 come from? Not from the inside of the bottle as ignorant 

 people might imagine, but from the surrounding atmosphere ; in 

 consequence of the capacity of the air to absorb and retain mois- 

 ture, increasing or diminishing, as its temperature grows warmer 

 or colder. 



Thus when the cold bottle is introduced into the room, the 

 warm sheet of air, which is in immediate contact with its surface, 

 immediately cools, and being no longer able to retain all the 

 moisture with -which it was impregnated, is obliged to deposit it 

 on the sides of the vessel. This familiar example suffices to 

 explain the formation of dew, rain, hail, snow, hoar-frost, and 



