486 Condition of the Moon's Surface. [October, 



density of any possible lunar atmosphere. But if we extend 

 our theories of lunar activity, so as to include changes 

 resembling those due to terrestrial vegetation, or again to 

 those which living creatures might produce by their works, 

 we are bound to take some thought of this relation. We 

 must, in fact, consider how far it is probable that any form 

 of vegetation, or any kind of life can exist, where the 

 atmospheric density is as small, let us say, as in the so- 

 called vacuum produced by one of our most perfect: air- 

 pumps. Now it appears unsafe to argue that no kind of 

 life, animal or vegetable, can exist in such an atmosphere 

 merely because our experience has not made us acquainted 

 with any. But, on the other hand, it must be remembered 

 as we proceed, that whatever degree of difficulty there may 

 be in admitting the existence of vegetable or animal life 

 under such conditions, is opposed to the occurrence of lunar 

 changes explicable as due to such forms of life. The whole 

 question being one of probabilities, we must not overlook 

 this antecedent improbability. 



At the same time, life exists under such varied conditions 

 on our own earth, that it is impossible to assert that, where 

 there is certainly very little air, and as certainly very little, 

 if* any, moisture, life cannot exist. Let us admit the possi- 

 bility, and let us further admit that the strange vicissitudes 

 to which living creatures on the moon would be exposed 

 during the lunar day and night, are not necessarily fatal to 

 the hypothesis that life exists on the moon. 



We have, then, two forms of change to enquire into — 

 those due to mechanical, chemical, and other like processes, 

 and those due to the existence of life upon the moon. 



But at the outset of the enquiry, we must take into 

 consideration a circumstance which is very frequently 

 overlooked in dealing with this subject. In all terrestrial 

 comparisons to determine processes of change, the observer 

 or experimenter is careful always to keep the circumstances 

 unchanged under which the object of research is examined. 

 If he desired to ascertain whether some distant and (let us 

 say) inaccessible surface underwent changes, he would, to 

 speak plainly, be careful to look at that surface in the same 

 way throughout his experiments, and also to select occasions 

 when the atmosphere was in some given condition. 



Now, first, the conditions under which any lunar object is 

 observed necessarily change with the progress of the lunar 

 day. As the sun gradually rises higher and higher above the 

 horizon of any lunar place, the shadows not only decrease 

 in length, but shift in direction ; and as the sun passes 



