Transportation on Water 379 



north? 7. What discovery did Columbus make in regard to the 

 variation of the needle? 8. Where now is the line of no variation? 

 In what direction is it moving? 9. Describe how you can determine 

 a north and south line by astronomic observations. 10. How can 

 you determine the variation of the needle for your locality ? 



11. What difficulty appears in the use of the magnetic compass 

 on steel ships? 12. (a) How can the earth's magnetism be used to 

 magnetize an iron or steel bar? (6) When was this method known? 

 (c) What was William Gilbert's explanation of the action of the com- 

 pass or other magnetized bodies? 



13. What modern device has taken the place of the magnetic com- 

 pass on many steel ships ? Describe. 14. Of what use is the chronom- 

 eter? On what time is it run? 15. How is the ship's latitude deter- 

 mined? 16. (a) Whafis the log? (6) How is it operated? (c) What 

 does it show? 17. What is meant by " dead reckoning "? 



VII. HARBORS 



170. Improvement of waterways. On the sea man must 

 adapt himself and his appliances to the conditions of wind 

 and water as he finds them. He charts his ways to avoid ob- 

 stacles such as icebergs, reefs, and shoals, and to suit the pre- 

 vailing direction of current and wind. With inland waters, 

 however, man works changes that facilitate commerce; he 

 builds harbors, dredges river channels, and constructs canals 

 by which natural waterways are joined ; and he builds boats 

 that are specially adapted to the conditions and needs. 



171. Importance of harbor improvement. It is certain 

 that inadequate harbor facilities have retarded to a consider- 

 able extent the construction of large vessels and the develop- 

 ment of shipping commensurate with the demands. By 

 enlarging and improving harbors shipping is stimulated and 

 greatly increased ; and so important is this that harbor cities 

 and the Federal Government unite in the work. It is recog- 

 nized that with channels sufficiently wide and deep, and with 

 piers sufficiently capacious and strong, and equipped with 

 modern machinery for loading and unloading, the largest ships, 

 now only contemplated, will become realities. 



