RADIATION IN THE SOLAR SYSTKM. 193 



(he earth is prolonii-cd at its ])ros(Mit tcnipiM-atiii-c to liiiii<lr(Mls of bil- 

 lions of years. 



l)iit here aiiiiiii si/(> is o,verythin<>-. Reduce the diameter of the 

 luoviny hody. and the retarding effect increases in proportion to the 

 i'e(hicti()n. If the earth ^Yere reduced to the size of a marble, the 

 effect would he appreciable in a hundred thousand years. If it were 

 rechiced to a spcck of dust a tliousandth of a centimeter in diameter, 

 the effect would be ajipreciable in a hundred years. 



Note what the elfect wouhl be. Imaiiine a dust particle shot out 

 rrt)m the earth and left behind to circuhite on its own account round 

 the sun. It would be heated by the sun and would be radiating out 

 on all sides. As it journeyed forward there would be a resisting 

 force teiuling to stop it. But instead of acting in this way the resist- 

 ance would enable the sun to pull the particle inward, and the fall 

 inward would actually increase the velocity. This increase in the 

 velocity would increase the resistance, and at the same time the 

 ai)})i"oach to the sun Avould raise its temperature, increase the radia- 

 tion, and so increase the resistance still further. The particle would 

 therefore move in a mere and more rapid orbit, and ultimately it 

 would fall into the sun. Small marble-sized meteorites would fall in 

 from the distance of the earth probably in a few million years. 

 Small particles of dust would be swept in in a few thousand years. 



Thus the sun is ever at work keeping the space round him free 

 from dust. If the particles are very minute he drives them forth into 

 outer space. If they are larger he draws them in. It is just possible 

 that we have evidence of this drawing in in the zodiacal light, that 

 vast dust-like ring which stretches from the sun outwards far beyond 

 the orbit of the earth and is at once the largest and the most myste- 

 rious member of the solar system. 



SM 1904 13 



