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 shallow- 
ness 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 cur- 
rent 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 water- 
way 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.