9928 . | DYNAMICAL GEOLOGY. — [Boox TIL. | 
ripples to huge billows. Long-continued gales from the seaward — 
upon an exposed coast indirectly effect much destruction, by the — 
formidable battery of billows which they bring to bear upon the 
land. Wave-action is likewise seen in a marked manner when wind ~ 
blows strongly across a broad inland sheet of water, such as Lake © 
Superior. (Section ii. § 6.) 2 
3. Alteration of the Water-level.—When the wind blows 
freshly for a time across a limited area of water, it drives the water 
before it, which is thus kept temporarily at a higher level, at the 
further or windward side. In a tidal sea, such as that which 
surrounds Great Britain, and which sends abundant long arms into 
the land, a high tide and a gale are sometimes synchronous, This 
conjunction causes the high tide to rise to a greater height than 
elsewhere in those bays or firths which look windward. With this — 
conjunction of wind and tide, considerable damage to property has ~ 
sometimes been done by the flooding of warehouses and stores, while — 
even a sensible destruction of cliffs and sweeping away of loose 
materials may be chronicled by the geologist. On the other hand, a — 
wind from the opposite quarter coincident with an ebb tide will 
drive the water out of the inlet, and thus make the water-level — 
lower than it should otherwise be. But even in inland seas where tides 
are small or imperceptible, considerable oscillations of water-level 
may arise from this action of the wind. At Naples for example a 
long-continued south-west wind raises the level of the water several — 
inches. In lorg fresh-water lakes also similar results attend pro- 
longed gales along the length of the lakes. 
Section ii.—Water. 
Of all the terrestrial agents by which the surface of the earth is 
geologically modified, by far the most important is water. We have 
already seen, when following hypogene changes, how large a share is 
taken by water in the phenomena of volcanoes and in other subter- — 
ranean processes. Returning to the surface of the earth and watch- 
ing the operations of the atmosphere, we soon learn how important a 
part of these is sustained by the aqueous vapour by which the atmo- 
sphere is pervaded. | 
The substance which we term water exists on the earth in three 
well-known forms—(1) gaseous, as invisible vapour; (2) liquid, as 
water; and (3) solid, as ice. The gaseous form has already been 
noticed as one of the characteristic ingredients of the atmosphere 
(p. 31). Apart from the heated reservoirs at the roots of volcanoes, 
it is in the air that this condition of the water-substance prevails. 
By the sun’s heat vast quantities of vapour are continually raised 
from the surface of the seas, rivers, lakes, snow-fields, and’ glaciers 
of the world, ‘This vapour remains invisible until the air containing 
it is cooled down below its dew-point, or point of saturation,—a result 
which follows upon the union or collision of two aerial currents 
of different temperatures, or the rise of the air into the upper cold 





