SOME UNSOLVED SCIENTIFIC PROBLEMS 



that the data regarding the sun alone were not suffi- 

 ciently definite. Thus Dr. Croll contended that if the 

 parent bodies of the sun had chanced to be " flying 

 stars " before collision, a vastly greater supply of heat 

 would have been engendered than if the matter merely 

 fell together. Again, it could not be overlooked that a 

 host of meteors are falling into the sun, and that this 

 source of energy, though not in itself sufficient to ac- 

 count for all the heat in question, might be sufficient to 

 vitiate utterly any exact calculations. Yet again, Pro- 

 fessor Lockyer called attention to another source of 

 variation, in the fact that the chemical combination of 

 elements hitherto existing separately must produce large 

 quantities of heat, it being even suggested that this source 

 alone might possibly account for all the present output. 

 On the whole, then, it became clear that the contraction 

 theory of the sun's heat must itself await the demonstra- 

 tion of observed shrinkage of the solar disc, as viewed by 

 future generations of observers, before taking rank as an 

 incontestable theory, and that computations as to time 

 based solely on this hypothesis must in the meantime be 

 viewed askance. 



But, the time controversy having taken root, new 

 methods were naturally found for testing it. The ge- 

 ologists sought to estimate the period of time that must 

 have been required for the deposit of the sedimentary 

 rocks now observed .to make up the outer crust of the 

 earth. The amount of sediment carried through the 

 mouth of a great river furnishes a clew to the rate of 

 denudation of the area drained by that river. Thus the 

 studies of Messrs. Humphreys and Abbot, made for a 

 different purpose, show that the average level of the 

 territory drained by the Mississippi is being reduced by 



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