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University of California Publications. [Geology 



meager data upon which the currents are charted. In the 

 preparation of the Pilot Charts, only actual reports by officers 

 erasing in the Pacific are considered and particular attention in 

 observing and reporting currents is requested of all mariners. 



The amount of current reported by a vessel is the difference 

 in the position of the vessel each day as obtained by dead reckon- 

 ing and the position obtained by astronomical observation. In 

 getting the position by dead reckoning, there are several sources 

 of error. Among these are the variation of the log in finding 

 the velocity of the ship, the failure of the man at the wheel to 

 hold the ship accurately to the prescribed course, and the un- 

 certainty as to the exact variation of the magnetic needle. This 

 last source of error may be of considerable amount. The North 

 Pacific Pilot Chart for February 1904 says: "Taking the 

 whole of the navigable world into consideration, it would be 

 conservative to state that the general uncertainty in the heading 

 of a ship at sea, arising from an inaccurate knowledge of the 

 variation of the needle, is as much as two degrees." This alone 

 would make a discrepancy of 15 to 20 miles between the calcu- 

 lated and the true position of a fast steamer at the end of each 

 day's ran. Turning to the other half of the problem, it is 

 found that a probable error of 2~y 2 miles must be allowed in 

 obtaining the true position by astronomical observation.* The 

 difference between the ship's position as determined by these 

 two methods with the possible sources of error is reported as 

 current or drift. It is well to note that the reports of steamers 

 have not the same value as those of sailing vessels. The sailing 

 vessel is occasionally becalmed for an entire day and then the 

 difference in position between two astronomical observations 

 gives a reasonably accurate determination of the drift. The 

 normal rate of a ship being known, it would seem as if it might 

 fully be allowed for, and hence the current might be determined 

 from a vessel under way, with an ecpial accuracy as from a 

 drifting ship. This would be true were it not for the fact that 

 ocean currents and drifts are not large masses of uniformly 

 moving water, but rather relatively narrow streams with bands 

 * Estimate by Professor George Davidson. 



