1887.] on the Sun's Heat. 15 



fringe of the fluid mass would probably be mucb less than the 

 distance fallen by each globe before the collision, because the trans- 

 lational motion of the molecules constituting the heat into which the 

 whole energy of the original fall of the globes become transformed in 

 the first collision, is probably about three-fifths of the whole amount 

 of that energy. The time of flying out would probably be less 

 than half a year, when the fluid mass mast begin to fall in again to- 

 wards the axis. In something less than a year after the first 

 collision the fluid will again be in a state of maximum crowding 

 round the centre, and this time probably even more violently agitated 

 than it was immediately after the first collision ; and it will again 

 fly outward, but this time axially towards the places whence the two 

 globes fell. It will again fall inwards, and after a rapidly subsiding 

 series of quicker and quicker oscillations it will subside, probably 

 in the course of two or three years, into a globular star of al30ut the 

 same dimensions, heat, and brightness as our present sun, but difiering 

 from him in this, that it will have no rotation. 



We supposed the two globes to have been at rest when they were 

 let fall from a mutual distance equal to the diameter of the earth's 

 orbit. Suppose, now, that instead of having been at rest they had 

 been moving in opposite directions with a velocity of two (more 

 exactly 1'89) metres per second. The moment of momentum 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 velocity 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 in all respects like to our sun. 



If instead of being at rest initially, or moving with the small 

 transverse velocities we have been considering, each globe had a 

 transverse velocity of three-quarters (or anything more than • 71) of 

 a kilometre per second, they would just escape collision, and would 

 revolve in ellipses round the centre of inertia in a period of one year, 

 just grazing each other's sui'face every time they came to the nearest 

 points of their orbits. 



If the initial transverse velocity of each globe be less than, but 

 not much less than, '71 of a kilometre per second, there will be a 

 violent grazing collision, and two bright suns, solid globes bathed in 

 flaming fluid, will come into existence in the course of a few hours, 

 and will commence revolving round their common centre of inertia in 

 long elliptic orbits in a period of a little less than a year. Tidal 

 interaction between them will diminish the eccentricities of their 



