164 FROM NEBULA TO NEBULA 



along, the comets have not failed to sustain their unen- 

 viable repute. Scientifically, they are generally looked 

 upon as strays from neighboring systems, and this char- 

 acter they undoubtedly possess, notwithstanding that 

 many of them have succeeded in insinuating themselves 

 as permanent occupants of our solar hearth. Their man- 

 ners it is that proclaim them strangers; they move ec- 

 centrically, redden when they near the sun, and pres- 

 ently turn tail and hurry away, perhaps never to return. 

 But what, in truth, are they, and why do they behave as 

 they do? 



Imagine two bodies of equal mass away off in space 

 and separated from each other by a considerable inter- 

 val, and, midway between them, a cube of lead and a 

 similar cube of ivory glued firmly face to face. Under 

 these circumstances, clearly the bi-cube would take up a 

 neutral position, disposing its longer axis at right angles 

 to the line joining the attracting bodies. Suppose, now, 

 one of the planets to remain stationary while the other 

 is violently projected straight toward it. In this case, 

 according to my interpretation, the bi-cube will incline 

 its lead end toward the nearing planet and, by the same 

 token, turn its ivory end toward the stationary one, more 

 and more, until, finally, the long axis instead of remain- 

 ing transverse, will run parallel with the line of force. 

 An unreflecting person, watching the whole proceeding, 

 might misinterpret this action of the double cube as 

 attraction upon the lead and repulsion on the ivory; but 

 we know better than that, because we know that ivory 

 by itself will fall as well as lead, so it too must be at- 

 tracted. Indeed, had we mated the ivory cube with a 

 cube of cork, instead of lead, it would have enacted the 

 same role as the lead. In other words, differential at- 

 traction due to variation in density creates the illusion 

 of repulsion. 



Let us carry the experiment a step farther and im- 

 agine, in lieu of the bi-cube, a sealed tube many miles in 

 length, nearly filled with equal volumes of a dozen mu- 

 tually insoluble liquids, like mercury, water and benzol, 

 and repeat the first process in the same order. Can you 



