273 
permeated by another and no less important one of watery * 
vapour, always present, though in varying quantities, accord- 
ing to circumstances of temperature, time, and place, derived 
from the ocean, the seas, lakes, pools, rivers, streams, — from 
the surface of the earth itself, and from all living things 
animal or vegetable. It is constantly rising and permeat- 
ing the air up to the point where saturation is reached, 
or until it is condensed by cold, into the sensible form of 
dew, clouds, or rain. On the varying conditions under which 
evaporation, on the one hand, and condensation on the other, 
take place, the rainfall depends. 
The ocean is the great source whence atmospheric moisture 
is derived ; it is the great bourne to which it all returns. As 
the wise king said, — “ All the rivers run into the sea, yet the 
sea is not full ; unto the place from whence the rivers come, 
thither they return again.” 
The atmosphere is the great sponge that soaks up and holds 
the watery vapour, which, when condensed, falls into the 
ocean, or on to the earth, to fill the rivers, to sink into the 
ground, whence it rises again in springs, collects in wells, 
lakes, and pools, or runs off in streams and rivers, diffusing 
itself everywhere, ministering to the wants of nature, and 
supporting life and organisation ; finally, to return to the 
ocean, again to rise in vapour, and repeat the endless circula- 
tion, without which life would be extinct, and the earth 
reduced to the condition of the moon, or of some effete worn- 
out world. 
Water is always evaporating ; expose a cup of it to the air 
and it will soon disappear, — all the sooner if the air be dry 
and warm. So will ice or snow, in regions where the cold 
may prevent it from melting, but not from evaporating ; it is 
not lost, but assumes the impalpable form of vapour, and 
mingles with the air. This process is going on wherever 
there is water, but more especially from that part of the ocean 
which, lying near the equator, is subjected to the continued 
heat of the vertical solar rays. Here vaporisation is most 
active, and the warm air, saturated with moisture, rising in 
constant currents to higher regions, is replaced by colder and , 
heavier currents rushing in from towards the poles ; in turn 
to be heated, charged with moisture, ascend, and so keep 
up a constant circulation, making the equatorial rain-belt the 
great distillery of nature. 
“ The wind goeth towards the south, and turneth about unto 
the north ; it whirleth about continually, and the wind re- 
turneth again according to its circuits.” These perennial 
northern and southern currents, or trade winds, getting their 
