ONE OF THE) JOYS OF SURVEYING 



Using a sled and four bullocks to transport instruments to the triangulation station at 

 White Rock, Virginia, which was situated in the mountains, miles from the railroad and 

 approached by no road. 



the Pacific coast, and in Hawaii, the 

 Philippines, and Alaska the datum is 

 lower, being "mean lower low water." 



THE WIRE DRAG 



The lead-line method of sounding- suf- 

 fices to record the lay of the bottom with 

 sufficient accuracy where there are no 

 extraordinary obstructions ; but in a re- 

 gion like the coast of Maine and that of 

 Alaska, where there are many isolated 

 pinnacle rocks and ledges under water, 

 or along shores like those of Florida, 

 Porto Rico, and the Philippines, where 

 coral reefs abound and coral heads 

 fringe the coast, special investigations 

 have to be made. The lead line might 

 be cast all around a pinnacle rock — ■ 

 might even strike it a glancing blow — 

 and still fail to discover it. 



An instance of this kind occurred in 

 Buzzard's Bay, Massachusetts, in 1902. 

 Although more than 91,000 soundings 

 had been made, more than 16,000 angles 



observed, and 1,462 miles of sounding 

 lines run, a rock whose head was 18 feet 

 below the surface was run upon by the 

 cruiser Brooklyn during the naval ma- 

 neuvers of that year. 



In order to discover such obstructions 

 in much frequented waters a new in- 

 strument, the wire drag, has been 

 devised. It consists of a long wire, 

 sometimes more than a mile long, 

 weighted down at intervals with sink- 

 ers and supported at any desired depth 

 by surface buoys. Several power-boats 

 are hitched to it, usually one at each end 

 and one in the middle, and with these 

 it is drawn around a harbor much as a 

 farmer drives his binder around his field 

 of standing wheat. If it strikes no ob- 

 struction the hydrographers know that 

 the harbor bottom is clear to the depth 

 of the drag (see page 656). 



Another line of information the mari- 

 ner must have is about the movement of 

 currents, so that his ship may not be 



660 



