760 Mr. HerUri Hall Turner [Feb. 1-' 



Right Ascension. Prediction of Declination. Prediction of 



points, and the time occupied by the planet in journeying from one 

 to the other in its orbit is nearly equal to that occupied by the 

 comet in its orbit : hence if both are near one crossing point 

 together, they will also l)e near the other point together. Jupiter 

 got behind the comet on both these occasions during the last round 

 and pulled it back, the direct effect of which is, of course, delay ; 

 l)ut an indirect effect overpowers this ; the comet does not go so 

 far afield when checked at the outset, and so comes home quicker. 

 When the juggler throws up his potatoes very vigorously they take 

 a long time to come down on to his forehead : but if something 

 were to check his throw, the interval would be smaller. 



As regards the hmit to which the comet attains, it journeys beyond 

 the orbit of Neptune, some thirty times the distance of earth from 

 sun, say 3000 million miles. It does not go far beyond this orbit, 

 but by the pecuUarities of elhptic motion under gravity it spends half 

 its time doing the small arc which hes beyond Neptune's orbit. 

 Attention was called earher in the lecture to the rapidity with which 

 gravity increases near the attracting centre ; as a consequence the 

 comet's movements are greatly accelerated, and it describes in a few- 

 weeks an arc equal to that over which it spends as many years at the 

 other extreme of its orbit. This lingering in the part of the journey 

 remote from the sun becomes more and more pronounced as the time of 

 revolution increases. We know that there are comets which take 

 thousands of years to return to the sun, instead of only 7o to no hke 

 Halley's comet : the greater part of this time they spend at a great 

 distance, travelling so slowly as to be almost stationary. Suppose 

 there were 1000 comets, each taking 1000 years to revolve round the 

 sun, of which about a year was spent in its neighbourhood (and within 

 our field of observation). This would provide us with about one 

 comet a year. If the period were 10,000 years we should require 



