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SCIENCE. 



[X. S. Vol. XXII. No. 556. 



offered information; and it is now a com- 

 monplace statement that Halley's questions 

 and suggestions caused Newton to write the 

 ' Principia. ' More than this, we know that 

 Newton's great treatise saw the light only 

 through Halley's persuasive insistence, 

 through his unwearying diligence in saving 

 Newton all cares and trouble and even 

 pecuniary expense, and through his abso- 

 lutely self-sacrificing devotion to what he 

 made an unwavering duty at that epoch 

 in his life. Again, he appears to have 

 been the first organizer of a scientific expe- 

 dition, as distinct from a journey of dis- 

 covery, towards the southern seas : he 

 sailed as far as the fifty-second degree of 

 southern latitude, devised the principle of 

 the sextant in the course of his voyaging, 

 and, as a result of the voyage, he produced 

 a general chart of the Atlantic Ocean, with 

 special reference to the deviation of the 

 compass. Original, touched with genius, 

 cheery of soul, strenuous in thought and 

 generous by nature, he spent his life in a 

 continuously productive devotion to astro- 

 nomical science, from boyhood to a span of 

 years far beyond that which satisfied the 

 psalmist's broodings. I have selected a 

 characteristic incident in his scientific ac- 

 'tivity, one of the most brilliant (though it 

 can not be claimed as the most important) 

 of his astronomical achievements ; it strikes 

 me as one of the most chivalrously bold 

 acts of convinced science within my knowl- 

 edge. It is only the story of a comet. 



I have just explained, very briefly, 

 Halley's share in the production of New- 

 ton 's ' Principia ' ; his close concern with 

 it made him the Mahomet of the new dis- 

 pensation of the astronomical universe, and 

 he was prepared to view all its phenomena 

 in the light of that dispensation. A comet 

 had appeared in 1682 — it was still the age 

 when scientific men could think that, by a 

 collision between the earth and a comet. 



' this most beautiful order of things would 

 be entirely destroyed and reduced to its 

 ancient chaos ' ; but this fear was taken as 

 a ' by-the-bye, ' which happily interfered 

 with neither observations nor calculations. 

 Observations had duly been made. The 

 data were used to obtain the elements of 

 the orbit, employing Newton's theory as a 

 working hypothesis; and he expresses an 

 incidental regret as to the intrinsic errors 

 of assumed numerical elements and of re- 

 corded observations. It then occurred to 

 Halley to calculate similarly the elements 

 of the comet which Kepler and others had 

 seen in 1607, and of which records had been 

 made ; the Newtonian theory gave elements 

 in close accord with those belonging to the 

 comet calculated from the latest observa- 

 tions, though a new regret is expressed that 

 the 1607 observations had not been made 

 with more accuracy. On these results he 

 committed himself (being then a man of 

 forty-nine years of age) to a prophecy 

 (which could not be checked for fifty- three 

 years to come ) that the comet would return 

 about the end of the year 1758 or the begin- 

 ning of the next succeeding year; he was 

 willing to leave his conclusion ' to be dis- 

 cussed by the care of posterity, after the 

 truth is found out by the event.' But not 

 completely content with this stage of his 

 work, he obtained with difficulty a book by 

 Apian, giving an account of a comet seen 

 in 1531 and recording a number of obser- 

 vations. Halley, constant to his faith in 

 the Newtonian hypothesis, used that hypoth- 

 esis to calculate the elements of the orbit 

 of the Apian comet; once more regretting 

 the uncertainty of the data and discounting 

 a very grievous error committed by Apian 

 himself, Halley concluded that the Apian 

 comet of 1531, and the Kepler comet of 

 1607, and the observed comet of 1682 were 

 one and the same. He confirmed his pre- 

 diction as to the date of its return, and he 



