THE BIRTH AND DEATH OF WORLDS 417 



of space they are doubtless cold. In their terrific rush through 

 the atmosphere they become incandescent. The amount of 

 heat they develop is proportional to their speed, squared. Their 

 speed is proportional to the attractive force of the earth. The 

 attractive force of the sun, at its surface, is twenty-eight times 

 greater than that of the earth ; the speed of the meteorites 

 through the atmosphere of the sun would correspond. The 

 surface of the sun is ten thousand of times that of the earth. 

 The quantity of meteorites swept up by this vast body would 

 be in proportion to its surface and to the force of solar gravity. 

 Mayer made some rough calculations, and in 1848, in a memor- 

 able paper, announced his Meteoritic Hypothesis. 



It was a noble effort ; but it did not hold. It was not long 

 before it became possible to make some calculations as to the 

 daily loss of the sun's energy by radiation. It is unthinkable. 

 You get a glimpse of the amount from an ingenious comparison 

 of Professor Langley's. He computed that if all the coal con- 

 tained in the earth could be burned in an instant, it would not 

 maintain the energy of the sun for one-tenth of a second. A 

 similar series of calculations made it clear that a quantity of 

 meteorites moving in space sufficient to keep up the sun's 

 outpour of heat, would mean such a bombardment of the earth 

 as to render the earth's surface red-hot. Obviously the meteor- 

 itic explanation was insufficient. The mystery remained. 



Five or six years later, Mayer's compatriot, von Helmholtz, 

 came forward with another explanation. By this time the once 

 fluid condition of the earth, suspected by Leibnitz and Buffon, 

 had been seemingly established. If the earth once existed as 

 a diffused fiery mass, it would, under the influence of gravity, 

 contract, just as Kant had supposed. In contracting it would 

 develop heat. This heat would be radiated away into space. 

 In the mind of von Helmholtz, here was the clue of the sun's 

 energy : contraction. 



The mass of the sun must once have occupied millions of 

 times its present volume. It must have been diffused through- 

 out a space extending perhaps thousands of times beyond 

 the present outermost limits of the solar system. By con- 

 traction it has shrunk to its present proportions ; by con- 

 traction is its energy maintained. This process is still going 

 on. But it is evident that it has finite limitations in time. 

 The mass of the sun is known ; the amount of contraction 



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