THE CENTURY'S PROGRESS IN ASTRONOMY 



because electrical force varies with the surface, while 

 gravitation varies only with the mass. From study of 

 atomic weights, and estimates of the velocity of thrust 

 of cometary tails, Bredichin concluded that the chief 

 components of the various kinds of tails are hydrogen, 

 hydrocarbons, and the vapor of iron ; and spectroscopic 

 analysis goes far towards sustaining these assumptions. 



But, theories aside, the unsubstantialness of the com- 

 et's tail has been put to a conclusive test. Twice during 

 our century the earth has actually plunged directly 

 through one of these threatening appendages, in 1819, 

 and again in 1861, once being immersed to a depth of 

 some 300,000 miles in its substance. Yet nothing dread- 

 ful happened to us. There was a peculiar glow in the 

 atmosphere, so the more imaginative observers thought, 

 and that was all. After such fiascoes, the cometary 

 train could never again pose as a world-destroyer. 



But the full measure of the comet's humiliation is, not 

 yet told. The pyrotechnic tail, composed as it is of por- 

 tions of the comet's actual substance, is tribute paid the 

 sun, and can never be recovered. Should the obeisance to 

 the sun be many times repeated, the train-forming mate- 

 rial will be exhausted, and the comet's chiefest glory will 

 have departed. Such a fate has actually befallen a mul- 

 titude of comets, which Jupiter and the other outlying 

 planets have dragged into our system, and helped the 

 sun to hold captive here. Many of these tailless comets 

 were known to the eighteenth-century astronomers, but 

 no one at that time suspected the true meaning of their 

 condition. It was not even known how closely some of 

 them are enchained, until the German astronomer Encke, 

 in 1822, showed that one which he had rediscovered, and 

 which has since borne his name, was moving in an orbit 



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