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Sect. III.] 



HYDROGRAPnY. 



61 



the course and rate of the current deduced for short 

 intervals of time instead of for the twenty-four hours. 

 The ohservations should commence previous to entering 

 the body of moving water, and be continued until after 

 the vessel has quitted it, when it will be advisable to 



occupy a page of the journal with a graphic delineation 



of the several courses of the stream, indicated by arrows, 

 and of the several stages of the vessel's progress by the 

 various temperatures which have been observed, noting 

 the places where ripples were seen, or where drift-wood, 

 sea-weed, or other floating substances occurred. 



The Stream or Surface Drift. 



7. Currents have been spoken of under the head of 

 " making a passage," as they affect a ship's route across 

 the ocean, and may have been determined by the position 

 of the sbip by DR differing from that by observation. 

 But it will be proper further to try the set of the surface 

 of the water on all favourable occasions, by the ordinary 

 method of anchoring, or of sinking a weight, endeavouring 

 if possible to get observations on the same day at about 



six hours apart, in order that it may be seen whetlier the 

 trcam be due to a tide or not. If the ship be in sound- 

 ings, and the day be calm, a very sim.ple way of effecting 

 this without the trouble of either anchoring or lowering a 

 boat,* is to drop a heavy lead from the quarter, and after 

 it has reached the bottom, to run out a small quantity of 

 stray line, and then make fast the " nipper," or a billet of 

 wood, to the line ; and at the same time to fasten the end 



* An objection to trying the current in a boat is the uncertainty of 

 the compass. 





tj- X — 



