ON COMETS. 139 



will have gone out to a distance 238 times the earth's 

 distance from the sun, or nearly 80 times the distance of 

 the planet Neptune. But this is still hardly the thou- 

 sandth part of the distance to the very nearest fixed star 

 and supposing the elliptical orbit of a comet should be 

 so long as to . carry it out only half-way to the nearest 

 star its return to the sun would require upwards of n 

 millions of years from its last appearance. Few of those 

 who saw the last-mentioned comet pass over Arcturus, 

 had any idea of the enormous distance at which the star 

 really was behind the comet : and Arcturus is by no 

 means the nearest star. 



(53.) I think, from what I have said, you will perceive 

 that there is in the history of comets matter enough both 

 to encourage inquiry and to check presumption. Looking 

 to the amount of our positive knowledge of them know- 

 ledge accfuired by centuries of observation, and by the 

 conspiring efforts within the last two centuries of the 

 profoundest thought and the most persevering labour of 

 which man is capable, we may reasonably enough con- 

 gratulate ourselves on what has been done, and while we 

 can -afford to look back with an indulgent smile on the 

 unfledged and somewhat puerile attempts of the ancient 

 mind to penetrate their secret, we may as reasonably 

 look forward to the revelations they will afford, as time 

 rolls on, of facts and laws of which at present we have no 

 idea. This may, and ought to inspire confidence of the 

 powers of man to penetrate always deeper and deeper 

 into the secrets of nature. But, on the other hand, here, 

 as on every other occasion, we find that the last and 



