7O A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



it is for a part of each month probably high enough to permit all the elements 

 which form our atmosphere to return to the vaporous or gaseous state. What, 

 then, is the condition which makes the moon an airless realm ? 



It appears to me that there is a possible explanation of the lack of an atmos- 

 phere on the moon, one that has not been subjected to the inquiry which it 

 deserves ; this is, in brief, that the kinetic movement of gases causes their atoms 

 to fly away from the surface into space as rapidly as they are parted from the 

 solid sphere. I understand that this hypothesis has been adduced to account for 

 the separation of certain gases from our atmosphere which are held in that 

 of the sun ; an extension of the same view may serve to explain the failure of the 

 moon to retain the gaseous materials which have evidently come to its surface 

 but which the gravitative attraction has not been sufficient to retain against the 

 diffusive effect of the kinetic movement. 



THE EXISTING CONDITION OF THE MOON. 



The idea that the moon should be the seat of some activities such as operate 

 on the earth is most natural. Again and again observers with much imagination 

 and with poor telescopes have seen what they took to be evidence of volcanic 

 action or of organic life on the surface. With the advance of selenography, these 

 views as to changes on the moon have been by better observations limited to two 

 groups of events. First, changes of form of certain craters, either those of a 

 cataclysmic and permanent nature, such as that which appears to have occurred in 

 the shape of the vulcanoid Linne, and the serial changes in certain other vulca- 

 noids, where the structures return to their original form ; second, the lightening 

 or darkening of color of certain patches of the surface as the lunar day advances. 

 There are also some assertions of minor alterations which need to be separately 

 considered. 



Of all the observations which point to the conclusion that changes are still 

 going on upon the moon, those which relate to the supposed sudden alteration of 

 Linne are the most important. This vulcanoid lies in the Mare Serenitatis, and 

 was mapped and described by several observers as having a crater about six miles 

 wide and with distinct steep walls. In 1866 it was believed that the structure did 

 not answer to the descriptions for in place of a crater there was found to be a 

 white spot of nearly twice its recorded diameter, and in the center of this field a 

 minute craterlet. Subsequent observations, however, have thrown doubt on this 

 conclusion, and led some selenologists to the opinion that Linne" is a structure 

 that varies much under diversities of illumination, and that its variations of 

 aspect, combined, perhaps, with some original bad mapping and servile copying, 

 may account for the seeming change. Other instances, which appear to indicate 

 the sudden appearance of craterlets where none were observed by skflled sele- 

 nographers, are easily accounted for by the same difficulties arising from the 

 conditions under which we behold the lunar surface. Thus it has been claimed 

 that the lava flow of the vulcanoid Mersenius, which on close scrutiny is seen not 



