THE SEMI-CENTENNIAL ANNIVERSARY 57 



of what has been done, I thought it might be as well to confine 

 myself to just a few illustrations of the kind of speculations 

 that we are at present being led to; and as these speculations, 

 mainly or wholly, also depend on the theory of star-streaming, 

 it may be well to begin by saying a few words about that theory. 



In order to get a clear idea of what is understood by the 

 phenomenon of star-streaming: Imagine two clouds or swarms 

 of stars, at first wide apart in space ; imagine that the stars within 

 each cloud move in all directions, indiscriminately, pretty much 

 as do the molecules of gas, and let us call this motion in the cloud 

 the " internal motion." In fact, imagine two immense gas bub- 

 bles, the molecules of which will be our stars. 



Now, imagine these two clouds to be moving in space, and 

 let that motion bring the two gas bubbles together, so that they 

 will penetrate each other. Then imagine that we, the spectators, 

 are in that part of the universe where the two bubbles have inter- 

 mixed, and finally imagine that we, the spectators, have a motion 

 of our own. 



What we will see of the motion of the individual gas mole- 

 cules will very nearly correspond to what we see of the motion 

 of the stars actually going on in the sky. 



Now, how does this motion show? Had the molecules in each 

 gas bubble no internal motion, that is, had they no other motion 

 than the common motion of all the molecules together as a whole 

 gas bubble, then of course what you would see would be this: 

 We would see two immense streams of stars, all moving in per- 

 fectly parallel lines, with perfectly equal velocity. If, however, 

 the internal motion is not zero, then of course what we will see 

 will be more or less different. The internal motion gives to each 

 molecule, in addition to the motion which is common to the whole 

 of the bubble, an additional individual motion, which will be 

 the cause of the total motion of the several molecules that are not 

 perfectly parallel or perfectly equal; and instead of seeing two 

 perfect streams with perfectly parallel motions, we must now see 

 the stars in the main parallel to two great directions; but there 

 will be deviations — small deviations will be frequent, greater 



