THE CONQUEST OF THE ZONES 



But more extended observation shows that this was 

 asking altogether too much of the compass, and while it 

 may be useful as an accessory it is by no means the 

 navigator's chief reliance in determining his location. 

 This is accomplished, as everyone is aware, in clear 

 weather by the observation of the heavenly bodies. 

 In cloudy weather, however, such observations ob- 

 viously cannot be made, and the seaman must direct 

 his ship and estimate his location — an all important 

 matter when he is approaching the coast — ^by what is 

 called dead reckoning. One element of this reckoning 

 is furnished by the compass, inasmuch as that is his 

 sole guide in determining the direction of the ship's 

 progress. The other element is supplied by the log 

 which furnishes him a clue as to the distance traversed 

 hour by hour. 



It is rather startling to reflect that the navigators of 

 the middle ages had no means whatever of determining 

 the rate of progress of a ship at sea, beyond the crudest 

 guesses unaided by instrument of any kind. When 

 Columbus made his voyage he had no means of know- 

 ing what distance he had actually sailed; nor was any 

 method of measuring the ship's speed utilized through- 

 out the course of the ensuing century. In the year 

 1570, however, one Humfray Cole suggested a the- 

 oretical means of measuring the ship's rate of progress 

 by means of an object dropped back of the ship and 

 allowed to drag through the water; and this suggestion 

 led a generation later to the introduction of the log, 

 which was first actually tested, so far as can be learned, 

 in the year 1607. 



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