Marvels of the Universe 



369 



the earliest times down to the end of 191 1 is nearly 

 eleven hundred, and the orbits of about five hundred 

 of these ha\"e been calculated. It is obvious, how- 

 ever, that the number which have appeared during 

 the last two thousand years is vastly more than the 

 eleven hundred just mentioned, because before the 

 invention of the telescope none of that class of 

 Comets which we call " telescopic " were observed, 

 although, doubtless, hundreds of such came into our 

 system and went away again. This fact will be readily 

 grasped when I state that 191 1 produced eight Comets, 

 of which no more than probably one would have 

 attracted the notice of the few astronomers or historians 

 who chronicled Comets in pre-telescopic times. Even 

 the number of eight belonging to the year ion, large 

 as it may seem, is not unprecedented : for 1898 pro- 

 duced ten Comets, 1886 produced nine, 185S produced 

 eight, as did 1881, and, not to multiply instances, 

 there have been in several recent years as many as 

 seven Comets each year. 



A subject of some interest is the absolute dimen- 

 sions of Comets as regards both heads and tails, 

 measured in miles. This is a matter in which we 

 meet with great extremes. 



The nucleus of the Comet of 1806 was esti- 

 mated to be only thirty miles in diameter, whilst 

 the nucleus of the third Comet of 1845 had a 

 diameter of eight thousand miles. So likewise as 

 regards the heads, or coma. The fifth Comet 

 of 1847 had a small head, only eighteen thousand 

 miles across, whilst the first Comet of 1811 had a 

 diameter of more than a million miles. But it is in 

 the case of the tails of Comets that we come across 

 the most stupendous figures. Whilst these may show 

 such moderate dimensions as only hundreds of thou- 

 sands of miles, the tail of the Great Comet of 1843 

 was calculated to be two hundred millions of miles 

 long. 



As a last word, let me say that cometary astronomy 

 is a branch of the science well worthy of the attention 

 of amateur students of nature ; for Comets, as will be 

 readily understood from what has gone before, are 

 constantly coming and going ; the changes of size and 

 form which they often go through are very curious 

 and diversified, and a large number of Comets may 

 be studied with the aid of the telescopes of moderate 

 size, which are now so common, and comparatively 

 cheap, considering the prices of half a century or 

 more ago 



THE DAYLIGHT COMET. 



As seen in North Africa in January. 1911. 



HALLEY'S COMET. 



Edmund Halley suggested the identity of the great 

 Comets of 1531. 1607 and 1682, and predicted a 

 reappearance at intervals of reventy-six years. 



