THE CONQUEST OP THE ZONES 



were he to overlook the latter the former would serve 

 but a vague and inadequate purpose. Yet, as just 

 indicated, this invaluable adjunct to the equipment 

 of the navigator was not available until well toward 

 the close of the eighteenth century. But of course 

 numerous general tables had been in use long before 

 this, else — to revert to the matter directly in hand — 

 it would not have been possible to make the above- 

 recorded test in the case of Harrison's famous watch 

 in the voyage of 1761-62. 



ASCERTAINING THE SHIP'S LONGITUDE 



In the days before the chronometer was perfected, 

 almost numberless methods of attempting to determine 

 the longitude of a ship at sea were suggested. There 

 were astronomers who advocated observation of the 

 eclipse of Jupiter's satellites; others who championed 

 the method of so-called lunars — that is to say, calcula- 

 tion based on observation of the distance of the moon 

 at a given local time from one or another of certain fixed 

 stars arbitrarily selected by the calculator. Inasmuch 

 as the seaman could always regulate even a faulty 

 watch from day to day by observation of the meridian 

 passage of the sun, it was thought that these observa- 

 tions of Jupiter's satellite or of the moon would serve to 

 determine Greenwich time and therefore the longitude 

 at which the observation was made with a fair degree 

 of accuracy. But in practice it is not easy to observe 

 the eclipse of Jupiter's satellite without a fair telescope ; 

 and it was soon found that the tables for calculating 



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