PROGRESS OF MODERN ASTRONOMY 



body of the comet, but yields to it in the case of gases, 

 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 spectro- 

 scopic analysis goes far towards sustaining these 

 assumptions. 



But, theories aside, the unsubstantialness of the 

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

 during the nineteenth century the earth has actually 

 plunged directly through one of these threatening ap- 

 pendages in 1819, and again in 1861, once being im- 

 mersed to a depth of some three hundred thousand 

 miles in its substance. Yet nothing dreadful hap- 

 pened to us. There was a peculiar glow in the atmos- 

 phere, so the more imaginative observers thought, and 

 that was all. After such fiascos 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 

 material will be exhausted, and the comet's chiefest 

 glory will have departed. Such a fate has actually be- 

 fallen a multitude of comets which Jupiter and the 

 other outlying planets have dragged into our system 

 and helped the sun to hold captive here. Many of 

 tailless comets were known to the eighteenth- 

 century astronomers, but no one at that time suspected 



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