252 



HISTORY OF SCIENCE. 



The problem of determining the longitude of a ship at sea is one 

 of the greatest importance to navigation. It resolves itself into ob- 

 serving some astronomical occurrence, the time at \vhich this occur- 

 rence is observed being counted according to the time of noon at the 

 place ; and this is compared with the hour at which the event takes 

 place according to the time at some given place, as Greenwich, for 

 example. All the practical methods of finding the longitude at sea 

 depend either on carrying the time from the first meridian, or on ob- 

 servations on the position of the moon. If the observer can find the 

 true place of the moon's centre, that is, its apparent position when 

 viewed from the earth's centre, he may calculate from tables what is 



FIG. 122. GREENWICH OBSERVATORY, 



the time at the first meridian at that instant. The method of finding 

 the longitude by observation of the moon's place had occurred to 

 several astronomers as early as the sixteenth century; but without good 

 tables of the moon's distances, and good means of observation, the 

 idea was of no available utility. And these difficulties were not sur- 

 mounted till the middle of the eighteenth century. At that time as- 

 tronomy had made such progress by improvements of the theory of 

 the moon's motions, and by the accumulation of correct observations, 

 that Mayer, who was excellent at once as a mathematician and an 

 astronomer, was enabled to draw up tables sufficiently accurate for 

 determinations of the longitude at sea. For this service Mayer's widow 

 received from the British Government the reward of ^3,000, a sum 

 which does not appear excessive when compared with the importance 

 of the results the tables were capable of furnishing. These tables, 

 however, removed only one part of the difficulty : there still remained 

 that of observation. Several of the methods proposed were too diffi- 



