ON COMETS. 117 



the appearance of a ground glass lamp the light always 

 becoming fainter and fainter, till it at last seemed to pass 

 awa.y from view from mere faintness. All this while, 

 however, there was a sort of smaller and much brighter 

 interior comet visible, with a tail-like appendage, which 

 seemed to be as it were a conducting channel by which 

 the matter of the newly-forming head was gradually re- 

 treating back into the centre. 



(29.) The discovery of the periodical return of Halley's 

 comets forms an epoch in the history of their bodies. 

 Since that time a great many more have been ascer- 

 tained to return at regular intervals. I will mention some 

 of the most remarkable cases of this kind. 



(30.) In 1770a comet appeared which proved rebellious 

 to the then adopted system of calculation, wliich set out 

 with assuming the orbit to be a parabola. It very soon 

 appeared, by the calculations of M. Lexell, that the real 

 orbit was an ellipse, and that not a very eccentric one. 

 In fact, all the observations were perfectly consistent with 

 an ellipse nearly coincident with the plane of the earth's 

 orbit, of such dimensions as that the extreme excursion 

 from the sun would carry it over a little beyond the 

 orbit of Jupiter, and its nearest approach would bring it 

 within that of Venus the time of its revolution being 

 5^ years. Here was quite a new fact. All other comets 

 then known had run out to limits far beyond our system 

 ' since even Halley's, with its period of 76 years at its 

 greatest distance from the sun, passed very far beyond 

 the orbit of Saturn, the most distant planet then known, 

 and in fact beyond the two since discovered, Uranus and 



