i 99 , 2r ] Flamsteed' s Observations : Halley 253 



observations, executed with all the accuracy that his in- 

 strumental means permitted. 



199. Flamsteed was succeeded as Astronomer Royal 

 by Edmund Halley, whom we have already met with 

 (chapter ix., 176) as Newton's friend and helper. 



Bom in 1656, ten year,s after Flamsteed, he studied 

 astronomy in his schooldays, and published a paper on the 

 orbits of the planets as early as 1676. In the same year 

 he set off for St. Helena (in latitude 16 S.) in order to 

 make observations of stars which were too near the south 

 pole to be visible in Europe. The climate turned out to 

 be disappointing, and he was only able after his return 

 to publish (1678) a catalogue of the places of 341 southern 

 stars, which constituted, however, an important addition 

 to precise knowledge of the stars. The catalogue was also 

 remarkable as being the first based on telescopic observa- 

 tion, though the observations do not seem to have been 

 taken with all the accuracy which his instruments rendered 

 attainable. During his stay at St. Helena he also took 

 a number of pendulum observations which confirmed the 

 results obtained a few years before by Richer at Cayenne 

 (chapter vin., 161), and also observed a transit of Mercury 

 across the sun, which occurred in November 1677. 



After his return to England he took an active part in 

 current scientific questions, particularly in those connected 

 with astronomy, and made several small contributions to 

 the subject. In 1684, as we have seen, he first came 

 effectively into contact with Newton, and spent a good 

 part of the next few years in helping him with the 

 Principia. 



200. Of his numerous contributions to astronomy, which 

 touched almost every branch of the subject, his work 

 on comets is the best known and probably the most 

 important. He observed the comets of 1680 and 1682 ; 

 he worked out the paths both of these and of a number 

 of other recorded comets in accordance with Newton's 

 principles, and contributed a good deal of the material 

 contained in the sections of the Principia dealing wLh 

 comets, particularly in the later editions. In 1705 he 

 published a Synopsis of Comet ary Astronomy in which no I 

 less than 24 cometary orbits were calculated. Struck by * 



