$1 301204] If alky 255 



his observation of the transit of Mercifry in 1677. In 

 three papers published by the Royal Society he spoke 

 warmly of the advantages of the method, and discussed 

 in some detail the places and means most suitable for 

 observing the transit of 1761. He pointed out that the 

 desired result could be deduced from a comparison of 

 the durations of the transit of Venus, as seen from different 

 stations on the earth, i.e. of the intervals between the first 

 appearance of Venus on the sun's disc and the final dis- 

 appearance, as seen at two or more different stations. He 

 estimated, moreover, that this interval of time, which would 

 be several hours in length, could be measured with an 

 error of only about two seconds, and that in consequence 

 the method might be relied upon to give the distance of 

 the sun to within about T \-^ part of its true value. As the 

 current estimates of the sun's distance differed among one 

 another by 20 or 30 per cent., the new method, expounded 

 with Halley's customary lucidity and enthusiasm, not un- 

 naturally stimulated astronomers to take great trouble to 

 carry out Halley's recommendations. The results, as we 

 shall see ( 227), were, however, by no means equal to 

 Halley's expectations. 



203. In 1718 Halley called attention to the fact that 

 three well-known stars, Sirius, Procyon, and Arcturus, had 

 changed their angular distances from the ecliptic since 

 Greek times, and that Sirius had even changed its position 

 perceptibly since the time of Tycho Brahe. Moreover 

 comparison of the places of other stars shewed that the 

 changes could not satisfactorily be attributed to any motion 

 of the ecliptic, and although he was well aware that the 

 possible errors of observation were such as to introduce 

 a considerable uncertainty into the amounts involved, he 

 felt sure that such errors could not wholly account for 

 the discrepancies noticed, but that the stars in question 

 must have really shifted their positions in relation to the 

 rest ; and he naturally inferred that it would be possible 

 to detect similar proper motions (as they are now called) in 

 other so-called " fixed " stars. 



204. He also devoted a good deal of time to the stand- 

 ing astronomical problem of improving the tables of the 

 moon and planets, particularly the former. He made 



