FINDING THE WAY AT SEA. v . 9 



problem that Whiston, the same who fondly imagined Newton was 

 afraid of him, 1 suggested the use of bombs and mortars ; for which 

 Hogarth pilloried him in the celebrated mad-house scene of the Rake's 

 Progress. Of course Whiston had perceived the essential feature of 

 all methods intended for determining the longitude. Any signal 

 which is recognizable, no matter by eye or ear, or in whatsoever way, 

 at both stations, the reference station and the station whose longitude 

 is required, must necessarily suffice to convey the time of one station 

 to the other. The absurdity of Winston's scheme lay in the implied 

 supposition that any form of ordnance could propel rocket-signals far 

 enough to be seen or heard in mid-ocean. Manifestly the only signals 

 available, when telegraphic communication is impossible, are signals 

 in the celestial spaces, for these alone can be discerned simultaneously 

 from widely-distant parts of the earth. It has been to such signals, 

 then, that men of science have turned for the required means of de- 

 termining longitude. 



Galileo was the first to point out that the satellites of Jupiter sup- 

 ply a series of signals which might serve to determine the longitude. 

 When one of these bodies is eclipsed in Jupiter's shadow, or passes 

 out of sight behind Jupiter's disk, or reappears from eclipse or occul- 

 tation, the phenomenon is one which can be seen from a whole hemi- 

 sphere of the earth's surface. It is as truly a signal as the appear- 

 ance or disappearance of a light in ordinary night-signalling. If it 

 can be calculated beforehand that one of these events will take place 

 at any given hour of Greenwich time, then, from whatever spot the 

 phenomenon is observed, it is known there that the Greenwich hour 

 is that indicated. Theoretically, this is a solution of the famous 

 problem ; and Galileo, the discoverer of Jupiter's four satellites, 

 thought he had found the means of determining the longitude with 

 great accuracy. . Unfortunately, these hopes have not been realized. 

 At sea, indeed, except in the calmest weather, it is impossible to ob- 

 serve the phenomena of Jupiter's satellites, simply because the tele- 

 scope cannot be directed steadily upon the planet. But even on land 

 Jupiter's satellites afford but imperfect means of guessing at the 

 longitude. For, at present, their motions have not been thoroughly 

 mastered by astronomers, and though the Nautical Almanac gives 

 the estimated epochs for the various phenomena of the four satellites, 



a carpenter) received 20,000. This sum had been offered for a marine chronometer 

 which would stand the test of two voyages of assigned length. Harrison labored fifty 

 years before he succeeded in meeting the required condition. 



1 Newton, for excellent reasons, had opposed Whiston's election to the Royal Society. 

 Like most small men, Whiston was eager to secure a distinction which, unless sponta- 

 neously offered to him, could have conferred no real honor. Accordingly he was amusingly 

 indignant with Newton for opposing him. "Newton perceived," he wrote, " that I could 

 not do as his other darling friends did, that is, Team of him without contradicting him 

 when I differed in opinion from him : he could not in his old age bear such contradiction, 

 and so he was afraid of me the last thirteen years of his life." 



