718 



SCIENCE 



[N. S. Vol. XXXVIII. No. 986 



probable inferences about the course of 

 events that have made our system what 

 it is. 



Some additional considerations might 

 easily have been added, but as I have had to 

 give up the idea of giving a general re- 

 view of what has been done, I thought it 

 might be as well to confine myself to just 

 a few illustrations of the kind of specula- 

 tions that we are being at present led to; 

 and as these speculations, mainly or 

 wholly, depend on the theory of star- 

 streaming, it may be well to begin by say- 

 ing 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 a gas, 

 and let us call this motion in the cloud the 

 "internal motion." In fact, imagine two 

 immense gas bubbles, the molecules of 

 which will be our stars. 



Now, imagine these two clouds or bubbles 

 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 intermixed, and finally imagine that 

 we, the spectators, have a motion of our 

 own. 



What we shall see of the motion of the 

 individual gas molecules will very nearly 

 correspond to what we see of the motion 

 of the stars actually going on in the sky. 



Now, what is the appearance of such a 

 motion? Had the molecules in each gas 

 bubble no internal motion, that is, had 

 they no other motion than the common 

 cloud-motion of all the molecules together, 

 as a whole, then of course what we would 

 see would be this: We would see two im- 



mense streams of stars, all moving in per- 

 fectly parallel lines, with what, linearly, 

 must be perfectly equal velocity.^ If, how- 

 ever, the internal motion is not zero, then, 

 of course, what we shall see will be more 

 or less different. The internal motion gives 

 to each molecule, besides the motion 

 which is common to the whole of the 

 bubble, an additional individual motion, 

 which will make the total motion of the 

 several molecules diverge more or less from 

 perfect parallelism and perfect equality. 

 Instead of seeing two streams with per- 

 fectly parallel motions, we must now see 

 the stars in the main parallel to two direc- 

 tions, but there will be deviations — small 

 deviations will be frequent, greater devia- 

 tions will be rare, and very great devia- 

 tions will be decidedly exceptional. The 

 motion of the two individual bubbles will 

 still be clearly discernible. 



Now this is indeed what we observe in 

 the sky. We recognize in the star motions 

 two clearly defined preferential motions. 

 These directions make an angle of about 

 one hundred degrees. The stars are not 

 moving all in these directions. Small de- 

 viations are frequent; greater deviations 

 are somewhat rare; very great deviations 

 are decidedly exceptional. 



We may say that all investigations made 

 since the first announcement of star-stream- 

 ing in 1904 — investigations based on very 

 different materials — all agree in the estab- 

 lishment of these two preferential directions 

 of motion among the stars. We find them 

 in the brighter stars; we find them in the 

 fainter stars; they show in the swift-mov- 

 ing stars; they show in the slow-moving 

 stars. They betray their existence in the 

 radial motions as well as in the motion at 

 right angles to the visual ray. » 



In the interpretation of the facts, how- 



1 Throughout the address the motions are to be 

 understood as relative to the sun. 





