79 PHYSICAL GEOGRAPHY OF. THE SEA 
tatory movement is also variable; its power is often very great, 
but sometimes water-spouts pass over smail 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 
