62 A COMPARISON OF THE I'KATURES OF THE EARTH AXFJ THE MOON". 



meteors which burn in our atmosphere are so minute that falHng at their present 

 rate they would not have formed a dust coating had they accumulated on the 

 surface of the earth for all recorded geologic time, and that the larger masses, 

 such as now attain the ground, have been so rare that they would not of them- 

 selves form a coating. (i>) That the earth and moon, as members of the solar 

 system, and sharing in its motion through space, are now in a field where meteors 

 are prevalent in a measure that was not the case in earlier ages, so that the 

 moon's surface, though very ancient, has not been long enough exposed to such 

 in-fallings to have acquired a coating of them, (c) That we have entirely mis- 

 judged the antiquity of the moon, and that our reckonings, based on the law of 

 cooUng bodies and on the supposition that the planet and satellite were differ- 

 entiated from a common nebulous mass, are altogether erroneous. Of these 

 suppositions, that designated as (5 seems the least objectionable, though as before 

 noted it presents sundry difficulties. 



In considering the effects arisino- from the fall of bodies from the celestial 

 spaces upon the surface of the moon, we should take into account the fact that in 

 the present airless state of that sphere they would come upon its surface at 

 a very much greater velocity than when they break through the atmosphere of 

 the earth. Owing to the resistance of the aerial envelope of our planet, it is 

 doubtful if even the heavier meteorites have at the moment when they touch the 

 ground an average velocity above a thousand feet a second. Computations 

 which assign them a higher speed at the moment of contact are made doubtful 

 by the slight amount of their penetration into the soil. On the other hand, the 

 meteors which fall upon the moon must be moving at the average rate of at 

 least twenty miles a second, or about one hundred times as rapidly. Where they 

 impinge on the advancing side of the moon the rate would be much greater than 

 where they come upon it from the other or following side. 



The effect due to the great speed at which meteorites would usually fall 

 upon the moon cannot be accurately determined ; certain of them can, however, 

 within limits, fairly be conjectured. It is in the first place evident that so far as 

 the penetration of mass into the crust was concerned it should be very much 

 greater than on the earth. On the assumption which has been above made as to 

 the comparative velocities, it should often be about a hundred-fold as great as on 

 the earth. It is, however, to be noted that the increase in velocity would 

 lead to a proportionate increase in the evolution of heat due to the friction 

 of the penetrating mass in its passage through the materials it encountered and 

 to the shearing of its particles on one another. Assuming the rocks of the lunar 

 surface to have the average resistance of pumice, it seems evident that any 

 meteoric body such as we know to fall upon the earth would not only penetrate 

 to a great depth, but would probably be volatilized by the very high temperature 

 it would attain. We see that a certain amount of this action occurs even in the 

 relatively slight resistance which a meteorite encounters in passing through the 

 air. With a resistance sufficient to produce an effective shearing movement in a 

 meteor, such as would be encountered on entering matter of the solidity of 



