Recurring Changes in the Universe. 161 



a star 1000 times more remote than u Centauri, though moving 

 transversely to the observer at the rate of 100 miles per second, 

 would take upwards of 30 years to change its position as much 

 as V — in fact, that we should have to watch the star for a 

 generation or two before we could be certain whether it was 

 chanoinor its position or not. It is evident that in regard to 

 the motions of the stellar masses, some attempt nnnt be made 

 to realize their dimensions in order to have a just appreciation 

 of the case. A motion that might seem harmonious and even 

 slow in the case of a great ship (for examp^) would be utterly 

 discordant and unfitting in the case of a child's toy boat. A 

 stellar mass that moved through a fractional thousandth part 

 of its own diameter in a second might be regarded as having 

 a majestically slow motion ; and yet if its diameter were a 

 million miles [which may possibly be an average value], it 

 would have traversed a thousand miles in that brief interval of 

 time. These considerations may serve to show that if we 

 apply the kinetic theory to the motions of the stellar masses, 

 how completely different the case is from an ordinary gas as 

 regards sequence of changes. The molecules of a gas, so far 

 from merely traversing a space comparable to their own diame- 

 ters in a second, are known as an actual fact to traverse a space 

 equal to countless millions of times their own mean distance 

 in a second [making roughly about ten thousand million col- 

 lisions a second] ; and yet, in spite of this rapid sequence of 

 changes, the molecules, as regards absolute velocity, are almost 

 at rest compared with the stellar masses. The scale is so incom- 

 parably different in the two cases*. Nevertheless, from the fact 

 that the energy (heat) developed at the collisions depends on the 

 absolute velocity, and of course not on the relative change of po- 

 sition of the masses, it becomes thereby possible by a sufficient 

 scale to have an absolute velocity of any value however high, 

 adequate to an enormous development of heat and explosive 

 rebound | at the encounters, combined with so small a relative 

 motion (or relative change of position) that the masses appear 



universe upon each other. We, on the other hand, are led to take the 

 diametrically opposite view, and to look to the dynamical acticn of the 

 moving parts of the universe upon each other as the sole means of ensuring 

 stability. 



* While in the case of a gas the scale is too small for us to overlook 

 directly the changes taking place in a unit of volume, in the case of the 

 universe the scale is too big. 



t The rebound would be far more than " explosive " in the ordinary 

 sense of that term, since the expansive action of the heat developed by 

 collisions even at moderate planetary velocities (calculably) far outrivals 

 both in energy and suddenness the case of explosives. Indeed, if the col- 

 liding masses were formed of gunpowder, its ignition at the collision 

 would add but little to the explosive energy of the recoil. 



