THE STORY OF NINETEENTH-CENTURY SCIENCE 



and which by jostling one another or through electrical 

 action become luminous. So widely are the individual 

 particles separated that the cometary body as a whole 

 has been estimated to be thousands of times less dense 

 than the earth's atmosphere at sea-level. Hence the 

 ease with which the comet may be dismembered and its 

 particles strung out into streaming swarms. 



So thickly is the space we traverse strewn with this 

 cometary dust that the earth sweeps up, according to 

 Professor Newcomb's estimate, a million tons of it each 

 day. Each individual particle, perhaps no larger than 

 a millet seed, becomes a shooting-star or meteor as it 

 burns to vapor in the earth's upper atmosphere. And 

 if one tiny planet sweeps up such masses of this cosmic 

 matter, the amount of it in the entire stretch of our sys- 

 tem must be beyond all estimate. What a story it tells 

 of the myriads of cometary victims that have fallen prey 

 to the sun since first he stretched his planetary net across 

 the heavens. 



in 



When Biela's comet gave the inhabitants of the earth 

 such a fright in 1832, it really did not come within 

 fifty millions of miles of us. Even the great comet 

 through whose filmy tail the earth passed in 1861 was 

 itself fourteen millions of miles away. The ordi- 

 nary mind, schooled to measure space by the tiny 

 stretches of a pygmy planet, cannot grasp the import of 

 such distances ; yet these are mere units of measure 

 compared with the vast stretches of sidereal space. 

 Were the comet which hurtles past us at a speed of, 

 say, a hundred miles a second to continue its mad flight 

 unchecked straight out into the void of space, it must fly 



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