658 



THE NATIONAL GEOGRAPHIC MAGAZINE 



appeared, creating a second channel. 

 Since that time it has moved two miles in 

 a northwesterly direction, and finally has 

 closed up the northern channel. 



OWING TO WINDS, TIDES, AND RIVERS, THE 

 WORK IS NEVER COMPLETED 



It is obvious therefore that the work 

 of surveying our coast-line can never be 

 finished, because the winds, the tides, and 

 the rivers continually alter its minor de- 

 tails, thereby making new investigations 

 necessary. These changes must be kept 

 track of and charted and the mariners 

 warned against them, else, confiding in 

 the accuracy of the charts, they would 

 literally be led into unsuspected traps. 

 The location of all lighthouses, buoys, 

 artificial and natural objects on the shore 

 must be charted, and every other aid to 

 navigation that will tell the skipper of 

 any ship exactly "where he is at" and 

 keep that information at hand all the 

 time. Piers and deepened channels 

 change the conformation of a harbor, and 

 sand-bars rise up or disappear with con- 

 siderable frequency. Without a proper 

 notation of these things, navigation would 

 be unsafe and insurance rates would be 

 high. 



Nor is this all. Marine architecture is 

 progressing, and with that progress ships 

 are growing longer, their waist-lines are 

 getting broader, and their drafts are be- 

 coming deeper. In 1848 the loaded draft 

 of the 20 largest ships in the world av- 

 eraged 19 feet. In 1873 the average of 

 the 20 largest ships was 24 feet, while 

 even in 1898 the average of the 20 largest 

 was only 29 feet. The length of the 20 

 largest ships in the world rose from an 

 average of 390 feet in 1873 to 640 feet 

 in 1903. It is obvious that a survey thor- 

 ough enough to meet conditions in 1873 

 would be wholly insufficient to meet con- 

 ditions in 1914. 



Once a rock that was 25 feet below 

 mean low water was of no interest to 

 navigators ; today three-fourths of our 

 navy and half of our shipping would be 

 in danger with such rocks uncharted. 

 There are thousands of rocks dotting the 

 under-water sections of our harbors and 

 shore-lines that could be neglected 20 

 years ago, but which today are great men- 



aces to navigation until they are located 

 and marked on the sailing charts. For 

 instance, the new super-dreadnought Nezv 

 York today would run aground in a thou- 

 sand places along the Atlantic coast where 

 the Oregon could navigate with impunity 

 in the day when it was the crack ship of 

 the American navy ; thus the Coast and 

 Geodetic Survey must always go deeper 

 with its investigation of the bottom of 

 navigable waters as the draft of ships in- 

 creases. 



a mariner's picture-book 



In short, the object of the coast survey 

 is to make a series of map pictures by 

 which navigators may read every detail 

 of coastal or harbor conditions that will 

 enable them to steer clear of all dangers. 

 These pictures must carry to the eye of 

 the mariner every feature of the shore- 

 line, every important feature of the bot- 

 tom over which his ship has occasion to 

 pass, as well as that over which dangers 

 forbid it to pass, and every detail of cur- 

 rent, tide, and compass behavior that en- 

 ables the mariner to keep in safe waters 

 and out of dangerous channels. 



On the map are located all the physi- 

 cal features of the neighborhood — its 

 high-water line, its low-water line, its ofT- 

 lying rocks, its streams, the elevations of 

 its hills, its towns, roads, lighthouses, 

 aids to navigation, church spires, tall 

 chimneys, peculiar rocks and trees, and 

 the like. 



MAKING THE SOUNDINGS 



In ascertaining the depth of the water 

 and locating all the under-water obstruc- 

 tions to navigation, a careful record of 

 the fluctuations of the tide while the 

 soundings are being made must be kept. 

 It would not suffice to measure the depth 

 of the water if its height above mean low 

 sea-level were unknown for the moment 

 of measurement. To determine this a 

 registering tide gauge is used — a sort of 

 float attached to a mechanism in which 

 a pen traces the rise and the fall of the 

 water on a roll of paper which a clock 

 causes to revolve under the pen. 



Two methods of sounding are used, the 

 one employing the lead line and the other 

 the wire sweep. In lead-line soundings 



