ON THE SUN'S HEAT. 409 



pose, now, that instead of having been at rest they 

 had been moving transversely in opposite direc- 

 tions with a relative velocity of two (more exactly 

 i '89) metres per second. The moment of momen- 

 tum of these motions round an axis through the 

 centre of gravity of the two globes perpendicular 

 to their lines of motion, is just equal to the 

 moment of momentum of the sun's rotation 

 round his axis. It is an elementary and easily 

 proved law of dynamics that no mutual action 

 between parts of a group of bodies, or of a single 

 body, rigid, flexible, or fluid, can alter the moment 

 of momentum of the whole. The transverse velo- 

 city in the case we are now supposing is so small 

 that none of the main features of the collision and 

 of the wild oscillations following it, which we have 

 been considering, or of the magnitude, heat, and 

 brightness of the resulting star, will be sensibly 

 altered ; but now, instead of being rotationless, it 

 will be revolving once round in twenty-five days 

 and so will be in all respects like to our sun. 



If instead of being at rest initially, or moving 

 with the small transverse velocities we have been 



