artificial channels of navigation. Thus man is compelled to extend over
the unstable waters the empire he had already founded upon the solid
land.

The upheaval of the bed of seas and the movements of water and of wind
expose vast deposits of sand, which occupy space required for the
convenience of man, and often, by the drifting of their particles,
overwhelm the fields of human industry with invasions as disastrous as
the incursions of the ocean. On the other hand, on many coasts,
sand-hills both protect the shores from erosion by the waves and
currents, and shelter valuable grounds from blasting sea-winds. Man,
therefore, must sometimes resist, sometimes promote, the formation and
growth of dunes, and subject the barren and flying sands to the same
obedience to his will to which he has reduced other forms of terrestrial
surface.

Besides these old and comparatively familiar methods of material
improvement, modern ambition aspires to yet grander achievements in the
conquest of physical nature, and projects are meditated which quite
eclipse the boldest enterprises hitherto undertaken for the modification
of geographical surface.

The natural character of the various fields where human industry has
effected revolutions so important, and where the multiplying population
and the impoverished resources of the globe demand new triumphs of mind
over matter, suggests a corresponding division of the general subject,
and I have conformed the distribution of the several topics to the
chronological succession in which man must be supposed to have extended
his sway over the different provinces of his material kingdom. I have,
then, in the introductory chapter, stated, in a comprehensive way, the