THE CONQUEST OP TIME AND SPACE 



for each hour of time, and fractions of the hour in that 

 proportion. 



It will be noted that this observation has value for the 

 purpose in question only in conjunction with certain 

 tables in which the movements of Jupiter and its satel- 

 lite are calculated in advance. This is equally true of 

 the various other observations through which the same 

 information may be obtained — as for example, the ob- 

 servation of a transit of Mars, or the measurement of 

 apparent distance between the moon and a given fixed 

 star. Before the tables giving such computations were 

 pubhshed it was quite impossible to determine the exact 

 longitude of any transatlantic place whatsoever. We 

 have already pointed out that Columbus had only a 

 vague notion as to how far he had sailed when he dis- 

 covered land in the West. The same vagueness ob- 

 tained with all the explorations of the immediately 

 ensuing generations. 



It was not until about the middle of the sixteenth 

 century that Mercator and his successors brought the 

 art of map-making to perfection; and the celebrated 

 astronomical tables of the German Mayer, which 

 served as the foundation for calculations of great im- 

 portance to the navigator, were not published until 1753. 

 The first Nautical Almanac ^ in which all manner of 

 astronomical tables to guide the navigator were included, 

 was published at the British Royal Observatory in 

 1767. 



At the present time, a navigator would be as likely 

 to start on a voyage without compass and sextant 

 as without charts and a Nautical Almanac. Indeed 



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