PRESSURE OF LIGHT IN ASTRONOMY 79 



in front than in the longer waves behind, and 

 therefore a greater pressure in front than behind ; 

 and the difference between them is a force directly 

 opposing the motion. 1 This force is found to be 

 proportional to the solar radiation, proportional 

 to the cross section of the particle and proportional 

 to its velocity. Its effect is that it is always draw- 

 ing energy out of the particle. The particle is, in 



FIG. 34. 



fact, always sending out more energy of radiation 

 than it receives from the sun, its expenditure is 

 greater than its income, and it is always drawing on 

 its capital, its own store of energy, to make both 

 ends meet. 



When a body moving round the sun is subjected 

 to a small resisting force taking out energy, it does 

 not, as we might at first thought expect, move more 



1 Note 7, p. 95- 



