i40 



NATURE 



{Dec. 6, 1888 



hence arrives at the view that "comets and meteorites may be 

 nothing else but one and the same phenomenon." 1 



This was in 1858, eight years before Schiaparelli's discovery. 



Newton, as we have seen, referred the comet of 1862 to the 

 largest meteorite in the August swarm. 



We may assume from the work which has already been done 

 that Reichenbach's view is more probably the true one, and 

 that the head of a comet is merely the denser part of the swarm. 

 Whether that denser part is at the end or at the beginning of the 

 Jong line to which reference has been made, it does not very 

 much matter, but where that is there we shall have the appear- 

 ance of a comet presented to us in the heavens. That being so, 

 we are able to apply everything that we have learned about 

 •comets to the movements of meteorites in space ; in the case of 

 meteors and falling stars we were limited to, what took place in 

 •our own air. 



The Appearar.ce' presented by Comets away from the Sun. 

 ■ When a comet first becomes visible, it appears in the tele- 

 •scope as a round misty body, and moves very slowly in conse- 

 quence of its still great distance from the sun. At this time, 

 too, its light is very feeble. Its appearance under these con- 



FiG. 12. — A comet near aphelion. 



■ditions strikingly resembles that of a nebula, and in fact comets 

 have often thus been mistaken for nebulae. 



Occasionally the appearance put on is that of a planetary 

 nebula in small telescopes and a globular one in larger ones. 



Fig. 13.— The Pons-Brooks coaiet, January 13, 1834 (Tholbn). 



The globular form, after a time, gives way, and the concentra- 

 tion of light is now a star-like concentration at one end of an 

 ■elliptic patch. 



' For this analysis of a part of Reichenbach's memoir, I am indebted to 

 my friend Mr. L. Fletcher, of the British Museum. 



In the next phase, both the star-like object and the elliptic 

 patch lengthen, and the appearance becomes more like what is 

 ordinarily recognized as a cornet^ 



As the comet approaches nearer the earth, so that observations 

 of its several portions may be seen, we get a still greater 

 differentiation of the' phenomena. 



■The first beginnings of a tail. 



Fig. l6,\ which is a representation of Donati's comet as it ap- 

 peared in 1858, will serve to illustrate the main characteristics of 

 comets. The brighter part is called the luad or coma, and some- 

 times there is within this a still brighter and smaller portion called 

 the nucleus. The tail\% the dimmer part radiating from the head. 



•'iG. 15.— The low-r portion represents the elongation of the star-like 

 luminosity ; the upper one. the oon^^cm tant t.xtens.on of the whole comet 

 (Comet 1882 October 25, Seabroke). 



and this varies greatly in different comets ; it may be long or short, 

 straight or curved, s ngle, double, or multiple. The comet of 

 1744 had six tails, that of 1823 two. In others the tail is 

 entirely absent. The tail of the comet of 1861 was 20,000,000 

 miles in length, and that of the comet of 1843 was 112,000,000 

 miles long. 



