COMETS 23 5 



They were not ; they were not deflected in the smallest 

 appreciable degree from their position and regular move- 

 ment ! One is naturally inclined to look upon the tail of 

 a comet as something like the smoke of a railway engine 

 trailing behind the advancing " head." As a matter of 

 fact, it does not always trail behind, but is always turned 

 away from the sun, so that when the comet is travelling 

 away from the sun the tail is in front ! It is now held 

 that the tail is caused by the radiant energy (light and 

 heat) of the sun, blowing, as it were, the lighter particles 

 from the incandescent head, and causing them to spread 

 out in a long track of variable shape. The photographs 

 of the third comet of the year 1908 show that the tail 

 can vary to an astonishing extent and with great rapidity 

 that is to say, in four or five hours. It is seen in those 

 photographs as a scimitar-like curved blade, then with a 

 second head or nucleus behind the leading one, then 

 actually bent like the letter Z, and then divided into 

 seven distinct diverging " plumes," and then it returns to 

 its former simple shape all in the course of a few days. 

 Astronomers have now shown that there is a close con- 

 nection between comets and the showers of "shooting 

 stars" or meteors which frequently strike the earth's 



comets, e.g. Wells (1882, ii.), whilst Holmes (1892) showed only 

 continuous spectrum. 



An interesting suggestion is made by Newall, namely, that the 

 spectrum is not indicative of the comet's composition, but of that of 

 the medium through which the body passes. Thus the persistent 

 identification of the cyanogen bands in cometary spectra is attributed, 

 primarily, to the " heating up " of cyanogen existing, free, in circum- 

 solar space. 



Till 1907 most of the cometary spectrograms showed only the 

 "carbon" and cyanogen radiations, but in Daniel's comet of that 

 year, and in Morehouse's of 1909, other lines were detected for which 

 origins have not, as yet, been found. 



Thus, some form of carbon + unknown + (occasionally) sodium and 

 iron seems to sum up our present knowledge of cometary composition. 



