360 POPULAR SCIENCE MONTHLY. 



they are not liquid, for no liquid could be suspended in sucli a 

 manner without being precipitated eventually upon the surface 

 of the planet. Nor are they solid ; for it has been demonstrated 

 that no solid could hold together under such strains, such tre- 

 mendous forces, as the attraction of the monster planet would 

 subject it to ; it would soon be broken up entirely. 



The only supposition remaining is that it is composed of 

 myriads of solid particles a ring of dust and fragments of rock 

 and stone. In this case we may imagine it as being an immense 

 swarm of tiny moons or satellites, each revolving in its own par- 

 ticular path around the planet, and the aggregation presenting to 

 us at this distance the appearance of a solid mass. Of course, the 

 word " tiny " must be taken in an astronomical sense, which 

 would not preclude one of these " dust " particles or fragments 

 from being as large as a house, or even a mountain. 



That the ring is composed of solid matter of some kind is 

 proved by the fact that it reflects the sunlight which it receives, 

 apparently unchanged in quality, and deprives of sunlight those 

 portions of the planet on which its shadow falls. But here comes 

 the question, If we know the ring is composed of solid matter, 

 how do we know that it is in the form of dust and fragments ? 

 This question was long a stumbling-block, but, as Prof. George 

 Darwin points out, the investigations of M. Roche, a French 

 mathematician, seem to have solved the difficulty. 



Briefly, the reasoning is as follows : We know that our moon 

 always keeps the same face toward the earth, but perhaps it is 

 not so generally known that the cause of this is in the moon's own 

 shape, which is that of an Qg^ with the longer diameter pointing 

 toward the earth. Not that this egg shape is so very pronounced, 

 but it is sufficient to keep the moon from rotating as the earth 

 does, and to keep its longer diameter pointed toward the seat of 

 that force which holds our satellite in its path. 



The cause of this egg shape is simply in what is termed the 

 " tide-generating force." The moon's efl^ect upon the earth due 

 to this force is rendered noticeable and well known in our tides. 

 The earth also exerts the same force upon the moon, only, as the 

 former is eighty times more massive, the effect is correspond- 

 ingly greater, and the moon's globe has suffered under the strain 

 has been pulled out of shape, so to speak. 



Now this force of course increases as its source is approached, 

 and were the moon brought nearer and nearer the earth, a point 

 might finally be reached where the solid materials of which she 

 is composed could no longer hold together, and her globe would 

 be torn to pieces by the tremendous forces to which she would 

 be subjected. To determine this point was the problem which 

 M. Roche solved, and his conclusions led him to place it at a 



