50 FROM NEBULA TO NEBULA 



rise to a great litter of loose detritus, which, by the wash- 

 ing back and forth of the tides, automatically arranges 

 itself in long and fairly regular ribs, which not only tra- 

 verse the ocean expanses but even, in a modified way, 

 invade the marshy areas as far inland as the arms of the 

 sea penetrate. 



THE MOON 



Whereas the surface of Mars is remarkable for its 

 smoothness, that of the moon is astoundingly rugged. 

 Why should this be so f It would be natural to suppose 

 that the difference in size might be the cause, but the 

 moment we include a third body, our earth, for the pur- 

 pose of comparison, we find this inference to be a mis- 

 take; for though much larger than either of the other 

 two, our planet is neither the first in roughness nor in 

 smoothness. Which of them constitutes the exception, 

 and why! My answer is, the moon. 



In casting about for points of difference and resem- 

 blance between these three bodies we cannot but be 

 struck by this one outstanding fact, namely, that whereas 

 the earth and Mars rotate in front of the sun in the space 

 of about 24 hours, the moon so rotates but once in 28 

 days. This peculiarity gives every point on her surface 

 a day one of our fortnights in length, followed by a 

 night of the same duration. In fact, her day is really a 

 summer and her night a winter. Now, there is no neces- 

 sary equivalence between her summer and her winter, 

 and one season is just as likely to overbalance the other 

 as v ice versa. This question only the resulting pheno- 

 mena can decide. 



Let us assume, then, the severity of the lunar winter 

 to overmatch the geniality of her summer, and reason 

 out what should be the natural effects; presupposing, 

 first, that her natural surface was originally as smooth 

 as that of Mars and, second, that she was once blessed 

 with her proportionate share of air and water. Imagine, 

 now, if you please, that during the first winter there fell 

 .on every foot of her land surface a depth of five feet of 



