120 ON COMETS. 



direction as the planets. It is but a small one, being 

 seldom visible without a telescope. Its orbit was first 

 computed on its appearance in 1795 (when it was dis- 

 covered by Miss C. Herschel), and again in 1805 and 

 1819. Upon this last occasion M. Encke, an eminent 

 computist, found that its motion could not be explained 

 without supposing it to move in an ellipse of the last 

 period I have mentioned and on searching back into 

 the records of comets he found those two I have just 

 named, which agreed perfectly, and proved to have been 

 really the same. 



(34.) Since that time it has been re-observed on every 

 subsequent revolution in '22, '25, '29, '32, '35, '38, '42, 

 '45, '48, '51, '55, and is always announced in the almanacs 

 as a regular member of our system. Its nearest approach 

 to the sun brings it just within the orbit of Mercury, and 

 on one occasion that planet happened to be so very near 

 it on its arrival, that it produced a pretty considerable 

 disturbance of the comet. But here, too, as in the case 

 of Lexell's comet, not the smallest perceptible effect was 

 produced by the comet on the planet; and thus two 

 valuable pieces of information were gained. First; As- 

 tronomers were enabled to estimate the mass or weight 

 of that small planet better than by any other means ; and 

 secondly ; It was proved that this comet also has no per- 

 ceptible weight and is also a mere puff of vapour, or 

 something as unsubstantial. 



(35.) There is another strange fact which this comet 

 has revealed. Its successive revolutions are each a little 

 shorter than the last a small fraction of a day, it is true, 



