THE SUN'S HEAT. 27 



compound body after the collision. Thus we see that the dry proba- 

 bility of collision between two of a vast number of mutually attracting 

 bodies widely scattered through space is much greater if the bodies be 

 all given at rest, than if they be given moving in any random directions 

 and with any velocities considerable in comparison with the velocities 

 which they would acquire in falling from rest into collision. In this 

 connection it is most interesting to know from stellar astronomy, aided 

 so splendidly as it has recently been by the spectroscope, that the rela- 

 tive motions of the visible stars and our sun are generally very small 

 in comparison with the velocity (G12 kilometres per second) a body 

 would acquire in falling into the sun, and are comparable with the 

 moderate little velocity (29*5 kilometres per second) of the earth in 

 her orbit round the sun. 



To fix the ideas, think of two cool solid globes, each of the same 

 mean density as the earth, and of half the sun's diameter, given at rest, 

 or nearly at rest, at a distance asunder equal to twice the earth's dis- 

 tance from the sun. They will fall together and collide in half a year. 

 The collision will last for a few hours, in the course of which they will 

 be transformed into a violently agitated incandescent fluid mass, with 

 about eighteen million (according to the Pouillet-IIelmholtz reckoning, 

 of twenty million) years' heat ready made in it, and swelled out by 

 this heat to possibly one and a half time, or two, or three, or four 

 times, the sun's present diameter. If instead of being at rest initially 

 they had had a transverse relative velocity of 1*42 kilometres per 

 second, they would just escape collision, and would revolve in equal 

 ellipses in a period of one year round the center of inertia, just graz- 

 ing one another's surfaces every time they come round to the nearest 

 points of their orbits. 



If the initial transverse component of relative velocity be less than, 

 but not much less than, 1*42 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 center of inertia in long 

 elliptic orbits in a period of a little less than a year. The quasi-tided 

 interaction will diminish the eccentricities of their orbits ; and if con- 

 tinued long enough will cause the two to revolve in circular orbits 

 round their center of inertia with a distance between their surfaces 

 equal to '644 of the diameter of each. 



If the initial transverse component relative velocity of the two 

 bodies were just sixty-eight metres per second, the moment of momen- 

 tum, the same before and after collision, would be just equal to that of 

 the solar system, of which seventeen eighteenths is Jupiter's and one 

 eighteenth the sun's ; the other bodies of the system being not worth 

 considering in the account. Fragments of superficially melted solid, 

 or splashes of fluid, sent flying away from the main compound mass 

 could not possibly by tidal action or other resistance get into the actual 



