Transportation on Water 387 



where fuel is obtained, foodstuffs are put into the markets, 

 building materials supplied where needed, and coal distributed 

 in regions where there is no natural supply. All this is done 

 at a cost that is among the lowest in the whole system of trans- 

 portation. 



Exercise : How locks are operated. The operation of a canal lock 

 may be made clear by a diagram illustrating a longitudinal section of a 

 lock (Fig. 137). Note these facts in your study of the diagram. A 

 boat enters through gate No. 1, while gate No. 2 is closed. At what 



FIG. 137. Diagram of canal lock. How is a boat lifted from lower water 

 level to upper water level? How are gates operated to let a boat down to the 

 lower water level ? 



level is the water in the lock? After the boat is in the lock, gate 

 No. 1 is closed and No. 2 is opened. What change is thus made in 

 the water level in the lock? 



Show by diagram how a boat is lowered from a higher to a lower 

 level of the canal. How are the gates operated for this? Describe 

 the passage of a ship from the third level (see illustration of Gatun 

 locks, Fig. 124) into the lake. Show how a ship passes from the lake 

 to the lower level of the canal. 



SUMMARY 



The improvement of channels, harbors, docks, and facilities for 

 handling freight makes possible the use of larger ships, insures their 

 safety, and necessitates only a brief stay in harbor. 



Navigable portions of the Mississippi and other rivers have been 

 improved by dredging, the building of levees and jetties, and the mark- 

 ing of channels. They furnish a relatively cheap method of trans- 

 portation. 



The Panama Canal, large enough for our naval vessels, is of great 

 defensive and commercial importance. It was begun by a French 

 company in 1861, and was completed by the United States in 1915, at 

 a cost of about $375,000,000. 



The completion of the canal was made possible in part by the dis- 



