72 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



which may have had this meteoric history, and also that Linne" is situated on a 

 wide mare where such stresses are indicated by the continuous ridges and 

 the rills, and where they would be more likely to accumulate than in the higher- 

 lying, irregular country. So if the supposed destruction of this vulcanoid really 

 occurred, a point which will ever remain doubtful, it may thus be accounted for 

 by other than volcanic action. It needs to be so explained if we are to retain our 

 conception of the moon as a sphere which has lost heat in the ratio that the earth 

 has lost it. 



The supposed variations in the shape of the twin craters known as Messier, 

 changes which appear to pass through something like a cycle in the course of a 

 lunar period, may possibly be due to the movement of extended masses of rock 

 under the influence of solar heat. Assuming, as before, that a sheet of rock on 

 one or more sides of the pits had, because of its expansion, developed a horizon- 

 tal joint a few feet below the surface, this slab-like mass might slide to and fro 

 with the variations of temperature. The expansion of a sheet fifty miles in 

 diameter might amount to several hundred feet, enough to make evident altera- 

 tions in the shape of the cavity. That some such migrations of rock masses 

 under terrestrial compressive strains are possible is abundantly proved by the 

 studies of geologists. Movements of ten miles or more are well ascertained ; the 

 only question is as to the possibility of a field of rock, such as we are considering, 

 returning, in the process of shrinking, to its original position. On the earth, such 

 a plate of stone would most likely be fractured as it cooled, so it could not return 

 to its first state. On the moon, however, such a mass, because its weight is 

 less than one-sixth what it would be on the earth, would encounter less friction in 

 its movements ; moreover, the grinding action of the adjacent surfaces would 

 tend to form a mass of powdery matter between them which would readily shear 

 so that the frictional resistance would be relatively small. The difficulties of 

 this hypothesis are obviously great, but if it is finally determined that there are 

 recurrent changes in process on the moon, such as appear to some observers 

 to take place in Messier, it seems preferable to that of volcanic action, for 

 it does not do violence to all we know concerning the processes of a cooling 

 sphere. 



We turn now to the changes of hue of certain fields of the lunar surface 

 such as have been observed by W. H. Pickering and others. These changes are 

 of two somewhat distinct kinds, those which appear to that observer to show the 

 discharge of fumes from certain small craters, and those which are thought pos- 

 sibly to indicate the temporary development of an extended vegetation which is 

 born in the brief season of a lunar day and dies in its night. As regards the 

 blotches of color which seem to indicate eruptions, I have had no chance to see 

 them, but from the account of the phenomena it appears most likely that they 

 are due to peculiarities of reflection much like those which make the rays glow 

 when the sun attains a high angle. The arguments against the existence of any 

 such clouds of vapor floating above the surface of the moon are very strong ; 

 they seem to me, indeed, to be insuperable. The phenomena of occultation 



