1878.] | Physiography. 673 
nature, the work of rain as an earth-sculptor is to cause the land 
to assume a gently undulating form, and to extend in dveadth those 
valleys which rivers are always tending to extend in depth. On 
those rocks, however, which are of a harder nature, rain has less 
absolute power, but even here it renders the scenery less rugged ; 
less sublime perhaps, but more beautiful. 
nd how comes this rain? We know that it falls from the 
clouds. We know too that these clouds are formed when the air 
above is cooled so much that it can no longer hold in solution all 
the vapor of water which it has borne in an invisible form from 
afar. The rain, therefore, comes from the vapor of water existing 
in the wind. And how comes it to exist in the wind? It is 
obtained from the Atlantic Ocean. Thither then we must travel 
in thought and try and picture to ourselves what takes place when 
the visible liquid water is converted into the invisible gaseous 
vapor of water. Now it is quite evident that some force is over- 
come—some binding force which drew the particles of water 
Closely together. This force is cohesion. It may be likened to a 
strong man who holds the watery particles in bondage, not indeed 
so severe as that of the terrible ice-king of the Arctic and Ant- 
arctic regions, for they are allowed free motion among each other 
and are not locked in the solid state, but still dondage chaining 
them down to the limits of the ocean. This strong man will not 
loose his grip until he be conquered by a stronger than he; and on 
the Atlantic he meets with that stronger man whom we call heat. 
Sun-heat sets free the particles of water from the bondage of 
cohesion, and allows them to escape into theair. But the mastery 
is not gained without an effort, and the value of this effort has 
been calculated. To emancipate nine pounds weight of water 
particles, an amount of energy has to be expended, equal to that 
of lifting a ton to the top of a precipice 2900 feet high? But just 
as, when two wrestlers struggle together, neither can master the 
other without a true waste of his substance taking place, a waste 
that has ere long to be made good by the absorption of a certain 
amount of mutton or beef, so too on the Atlantic, during the 
struggle between cohesion and heat, a certain amount of the lat- 
ter is consumed and disappears. The amount of heat so expended 
has also been calculated. In setting free nine pounds of water 
particles an amount of heat disappears sufficient to fuse forty-five 
pounds of cast iron : 
1 These are two different ways of stating the same fact. 
