NAVIGATION CURTISS. 135 



piloting. Before losing sight of land the longitude and latitude of 

 the last well-determined position found by methods of piloting is 

 taken from the coast chart and transferred to the ocean or small 

 scale chart and is considered as the point of departure. 



The point of departure is the starting point of the ocean voyage; 

 and from that point the course and distance are laid down, being 

 rectified whenever astronomical observations are available. More 

 accurately, though less vividly than on the chart, the changes in 

 longitude and latitude involved in each change of course are com- 

 puted by plane trigonometry, using so-called traverse tables for the 

 solution of the right-angled triangle involved. Such a method of 

 keeping account of a ship's position on the basis of the course as indi- 

 cated by the compass and the distance as indicated by the log, al- 

 lowing for wind, current, and tide, is called dead reckoning. As 

 bearing upon the accuracy of the log, it is interesting to note that 

 some authorities, in the case of steam vessels, consider that the revo- 

 lutions of the ship's propeller, taking into account the ship's draught 

 and the condition of the ship's bottom, afford the best means of esti- 

 mating speed. 



Astronomical reckoning affords the most accurate means of ascer- 

 taining positions at sea, dead reckoning being carried along partly 

 as a check but also to be relied on when weather does not permit 

 observations of the heavenly bodies. 



Navigators will generally prefer to determine position from ob- 

 servations of the sun, measuring the altitude of that body above the 

 sea horizon. But simillar observations of the planets and brighter 

 stars in twilight, when the horizon is well defined, afford even better 

 determinations of positions at sea. In such a case the careful navi- 

 gator, by observing for latitude two stars, one north and one south 

 of the zenith, and for longitude two stars, one east and one west of 

 the zenith can depend on a good result, especially if the stars in each 

 pair are about at the same altitude and not too low in the sky. Since 

 the moon also may be used when occasion arises, it is evident that the 

 navigator seldom needs. to go along without a good fix or determina- 

 tion of position. 



The chief astronomical observations made at sea are those for 

 ascertaining (1) latitude, (2) time and longitude, (3) compass error, 

 and (4) latitude and longitude simultaneously. 



The plan of many navigators is to observe with a sextant the 

 altitude of the sun in the morning when that body is nearly above 

 the east point, to determine local time and longitude. In the com- 

 putation of local time it is necessary to adopt a latitude obtained by 

 dead reckoning, but if the sun is well placed errors in the assumed 

 latitude will introduce relatively small errors in the resulting ship's 

 time. The longitude is obtained by taking the difference between the 



