80 PRESSURE OF LIGHT 



slowly. It falls inwards a little towards the sun, 

 and so yields up some of its potential energy. But 

 the potential energy thus yielded up is more than 

 that which the resisting force takes out, and the 

 balance is converted into energy of motion, and 

 the particle moves faster than it did. So that we 

 have as the result of this resistance to motion an 

 increase in the velocity, but in an ever-lessening 

 orbit. 



Calculation shows that a sphere of the density of 

 the earth, black so that it absorbs all the sun's 

 radiation falling on it, and I cm. radius, say the 

 the size of a marble, will fall in towards the sun, if 

 at the earth's distance from it, about 820 metres in 

 the first year. In successive revolutions, each tak- 

 ing less time than the last, it will fall in less, but in 

 successive periods equal to our year it will fall in 

 more ; and if it can be supposed to move in a 

 nearly circular spiral, always shortening its distance 

 by the same law, it will reach the sun in about 

 90,000,000 years. 



With smaller particles the action is more rapid 

 and a particle TTr Vrj- cm - in radius will reach the 

 sun from the distance of the earth in 90,000 

 years. 



To sum up these effects of light-pressure, we 

 have, first, the repulsion of exceedingly minute 

 particles which may be driven out of our system 

 altogether if they are sufficiently minute. On 



