A COMPARISON OF THE FEATURES OF THE EARTH AXD THE MOOX. 6l 



rather Improbable. While there is no recorded instance of any meteorite having 

 been found in ancient geological deposits or elsewhere, save upon the surface of 

 the earth, the rarity of falls sufficiently large to escape burning in the air makes 

 it unlikely that they would be discovered in a fossil state, or, if found, that they 

 would be recognized as of meteoric origin, so that this consideration has not 

 much weight. On the other hand, if, as seems likely, the supply of carbonic 

 dioxide in the air depends in any considerable measure on the burning in it of 

 carbon meteorites, the presence of this material in something like its existing 

 quantity, certainly neither much greater nor much less, from the early geologic 

 ages, is evidence that meteoric falls, at least those containing carbon and of the 

 smaller size, have during that time been at about the same rate as at present. So 

 far as I can discern the astronomic conditions, it seems very improbable that 

 the earth should now be encountering a multitude of small bodies such as had 

 not come to it until within a few thousand years. 



The suggestion that the meteoric matter which comes upon the earth may 

 have been expelled from it, though possible, does not seem to me to afford a way 

 of escape from our difficulty. It appears not improbable that volcanic action 

 may be sufficiently violent to impel bodies beyond the control of the earth's 

 attraction. The shining clouds which were observed for some years after the 

 eruption of Krakatoa in 1883, and which went upward until they appeared to 

 escape from the atmosphere, may be instances of this nature. Moreover, large 

 fragments, which have been hurled forth by great eruptions, have been known to 

 fall at such distances from the point of ejection as to make it likely that they had 

 an initial velocity near to that which would be necessary to send them into space 

 and to make them independent of the earth ; but, if I conceive the problem 

 rightly, such ejections would either in very rare instances fall upon the moon or 

 proceed to move in elliptical orbits, one focus of which would be the sun and 

 the other the place in space where the earth was at the time they separated from 

 it. It is eminently probable that in time these fragments would be apt to return 

 to the earth, but it seems evident that they would be about as likely to fall upon 

 the moon. 



If we had any evidence that the moon had been surrounded with a fairly 

 dense atmosphere down to the present geological period, we might account for 

 the absence of meteoric dust upon its surface by the supposition that the smaller 

 bodies had been burned in its air as they are in that of the earth, but all the facts 

 at hand, which will be discussed below, are distinctly against this supposition and 

 in favor of the view that the low gravitative value of the sphere allows the gases 

 which do not become solid at the low temperature which prevails there by 

 kinetic action to move off into space ; so that the development of an aerial 

 envelope has been impossible. 



I have but recently come upon the difficulties we have to face in this problem 

 concerning the preservation of the surface of the moon from meteoric matter, and 

 am therefore not well prepared to discuss them. As they now appear to me, they 

 may be met by any one of the following described hypotheses : («) That the 



