Masterpieces of Science 



system make it not wholly improbable that others 

 may not yet be found, especially in the systems 

 of Saturn and Neptune. But there seems very 

 little likelihood that satellites of Mercury or 

 Venus will ever be discovered, or any new at- 

 tendant of the earth. 



What is to be the progress of our knowledge in 

 respect to meteors and comets it is not easy to 

 --oresee. As regards their orbital motions there 

 is perhaps not much to expect, because our present 

 theory seems to be reasonably complete. And 

 yet it seems a priori not unlikely that the force 

 which operates to produce the tails of comets 

 should have some influence upon their move- 

 ments; and such a phenomenon as the persistent 

 acceleration of Encke's comet suggests, at least, 

 a possible necessity for farther refinements. 

 Certainly greater precision of observation is 

 needed to enable us to pronounce with certainty 

 upon the questions of cometary identity which 

 are continually arising. And these questions 

 are of extreme importance in their bearing upon 

 the theory of the origin of comets and their 

 relations to our system. We may earnestly 

 hope, therefore, that the surely growing accuracy 

 of observation and computation will throw 

 light upon this problem. 



As to the physical constitution and nature of 

 comets, we may, perhaps, expect a great im- 

 provement of our knowledge just because our 

 present ignorance is so great. Many facts, of 

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