OBSERVATION. 11 



There long appeared to be a strong presumption that 

 all comets moved in elliptic orbits, because no comet had 

 been proved to move in any other kind of path. The 

 theory of gravitation admitted of the existence of comets 

 moving in hyperbolic orbits, and the question arose 

 whether they were really non-existant or were only 

 beyond the bounds of easy observation. From reason- 

 able suppositions Laplace calculated that the probability 

 was at least 6000 to i against a comet which comes 

 within the -planetary system sufficiently to be visible at 

 the earth's surface, presenting an orbit which could be 

 discriminated from a very elongated ellipse or parabola, in 

 the part of its orbit within the reach of our telescopes 11 . 

 In short, the chances are very much in favour of our 

 seeing elliptic rather than hyperbolic comets. Laplace's 

 views have been confirmed by the discovery of six hyper- 

 bolic comets, which appeared in the years 1729, 1771, 

 1774, 1818, 1840, and 1843!, and, as only about 800 

 comets altogether have been recorded, the proportion of 

 hyperbolic ones is quite as large as should be expected. 

 Some remarkable speculations have recently been pub- 

 lished by Mr. A. S. Davies, as to the probable character of 

 the orbits of comets, which, after moving freely through 

 *pace, become attached to this planetary system k . 



When we attempt to estimate the numbers of objects 

 which may have existed, we must make large allowances 

 for the limited sphere of our observations. Thus pro- 

 bably not more than 4000 or 5000 comets have been 

 seen in historical times, but making allowance for the 

 absence of observers in the southern hemisphere, and 

 for the small probability that we see any considerable 



h Laplace, ' Essai Philosophique,' p. 59. Todhunter's ' History/ 

 pp. 491-94- 



i Chamber's 'Astronomy,' ist ed. p. 203. 



k 'Philosophical Magazine,' 4th Series, vol. xl. p. 190; vol. xli. p 44. 



