RADIATION IN THE SOLAR SYSTEM. 193 



the earth is pi-oloiined at its ])resent teinperatiiri' to hiindi'cMls of hil- 

 Jions of years. 



But here apiiu size is everythin<>\ llechice the diameter of the 

 nioviiiii' l)o<ly, and the retarding effect increases in proportion to the 

 reduction. If the earth were reihiced to the size of a marble, the 

 effect wouhl be aj)precial)le in a hunch-ed thousand years.. If it were 

 rcMbiced to a speck of (hist a thousandth of a centimeter in diameter, 

 the effect wouhl be a]:)])reciable in a hundred years. 



Note Avhat the eli'ect would be. Imagine a dust ])article shot out 

 from the earth and left behind to circulate on its own account round 

 the sun. It would be heated by the sun and Avould b(> i-adiating out 

 on all sides. As it journeyed forward there would be a resi'^ting 

 force tending 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 Avould actualh' increase the velocity. This increase in the 

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

 approach to the sun would 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. Snudl marble-sized meteorites would fall in 

 from the distance of the earth pi'obably in a few million years. 

 Small particles of dust would l)e 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 miiuite 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 aiul is at once the largest and the most myste- 

 rious member of the solar system. 

 sM 1904 13 



