The Use of Ships

195

used. These boats dg not sink into the water nearly so deep as do the
round-bottomed ships that sail the oceans and lakes. The shallowness
 of the water also makes it necessary to use paddle wheels on the
sides of the boats amidships, or in front, instead of propellers at the
stern. Paddle boats are slower than those with propellers. The current
 also causes river navigation to be slow upstream.
Canals can be dug in almost any level plain, but at so great cost
that it pays to build them only where there is sure to be a large amount
of freight. Canal traffic is always slow; for if fast steamboats were
used, the waves which they cause would soon wear away the banks
and fill the canals. Often barges are used in great numbers, and are
slowly drawn by tugboats, horses, donkeys, or even men. In densely
populated plains like those of China, Japan, and Europe, even such
slow transportation by inland waterways is important. If railroads
are also present, as along the Vistula in Poland, the canals are used
chiefly for bulky, non-perishable commodities such as ores, sand and
eravel, cement, and lumber.
Our most important inland waterways. The map suggests that
the Mississippi River should be the most important navigable waterway
 of North America aside from the Great Lakes. It is navigable
for large steamers and barges at all seasons to St. Louis, 1256 miles
from the mouth, and for smaller boats most of the year to St. Paul,

F1g. 139. Flat-bottomed boats that draw little water are used to haul barges in canals and
shallow rivers.