6 PITYSICAL GEOGRAPHY OF THE SEA. 
the eye by their monotonous uniformity; on others, where these 
natural bulwarks are wanting, artificial embankments, or dykes 
protect the lowlands against the encroachments of the sea, or else 
the latter forms vast salt-marshes and lagunes. On some coasts 
these submerged or half-drowned lands have been transformed 
by the industry of man into fertile meadows and fields, of which 
the Dutch Netherlands afford the most celebrated example ; while 
in other countries, such as Egypt, large tracts of land once cul- 
tivated have been lost to the sea, in consequence of long misrule 
and tyranny. 
How deep is the sea? How is its bottom formed? Does 
life still exist in its abyssal depths? These mysteries of ocean, 
which no doubt floated indistinctly before the mind of many an 
inquisitive mariner and philosopher of ancient times, have only 
recently been subjected to a more accurate investigation. Their 
solution is of the highest importance, both to the physical 
geographer, whose knowledge must necessarily remain incom- 
plete until he can fully trace the deep-sea path of oceanic 
currents, and to the zoologist, to whom it affords a wider in- 
sight into the laws which govern the development of the 
innumerable forms of life with which our globe is peopled. 
The ordinary system of sounding by means of a weight at- 
tached to a graduated line, and “armed” at its lower end with 
a thick coating of soft tallow, so as to bring up evidence of its 
having reached the bottom in a sample of mud, shells, sand, 
gravel, or ooze, answers perfectly well for comparatively shallow 
water, and for the ordinary purposes of navigation, but it 
breaks down for depths much over 1000 fathoms. The weight 
is not sufficient to carry the line rapidly and vertically to the 
bottom; and if a heavier weight be used, ordinary sounding 
line is unable to draw up its own weight along with that of the 
lead from great depths, and gives way, so that by this means no 
information can be gained as to the nature of the sea-bottom. 
To obviate this difficulty, several ingenious instruments have 
been invented, such as the “ Bull-dog” sounding machine, which 
is so contrived that on touching the bottom the weight becom s 
detached, while at the same time a pair of scoops, closing upon 
one another scissorwise on a hinge, and permanently attached 
