A COMPARISON OF THE FEATURES OF THE EARTH AND THE .MOOX. 63 



pumice, we may fairly assume that tlie mass would, in effect, explode, the gase- 

 ous products being cast forth from the opening it made. The temperature pro- 

 duced by the arrest of the movement at a rate of twenty miles a second would 

 vaporize the mass. 



It is also evident that on a surface in the present airless condition of the 

 moon all meteoric bodies, even the smallest, would come in contact with its rocks. 

 As is well known, by far the greater part of the meteors which enter upon the 

 earth are burnt in the upper air, and pass into the gaseous state or fall to the 

 ground gently in a purely divided condition. Such bodies, however minute, 

 would enter the moon's crust at the same speed as the larger masses. Owing, 

 however, to their smaller bulk, they would be more quickly dissipated by the 

 engendered heat. If this view as to the volatilization of meteors by the conver- 

 sion of the energy due to their motion into heat is true, then the effect of any 

 such meteoric fall as takes place on the earth in, say, a hundred thousand years 

 would be to produce a mass of gaseous and dust-like material which should be 

 somewhat widely scattered from the point of impact of each meteorite, and this 

 for the reason that the gases evolved by the heat would enter into what is essen- 

 tially a vacuum and would be radially distributed at high speed, quickly to fall 

 upon the ground as their temperature lowered. The effect of such action 

 would evidently be to give the lunar surface a uniform color, determined 

 by the average light-reflecting quality of the resulting deposits of condensed 

 vapors and dust. If, on the other hand, we assume that the material bodies 

 penetrated into the moon without being volatilized, then the result of the first 

 falls would be merely to pit the surface, the color being destroyed for the area of 

 each pit, but when the successively formed pits became so numerous that they 

 occupied the whole of the original area the color would disappear. The effect 

 can be the better realized by firing successive charges of shot at a white plank. 

 As the number of penetrations increases to a point where the total amount of 

 lead is equal to a continuous layer, the original material becomes, in effect, covered 

 with the metal and takes its hue. 



The considerations just above set forth make it appear eminently probable 

 that in either of the conditions in which we can imagine meteoric matter to have 

 come upon the moon, that in which it was vaporized or that in which it remained 

 solid, a period in a geological sense brief would suffice to obliterate the diversi- 

 ties of hue such as we find in the dark maria, the light streaks and patches, and 

 in its general surface. Thus the best interpretation which we can give to the 

 facts clearly leads to the supposition that our satellite has not in recent ages 

 shared with us in anything approaching like measure the falls of detached masses 

 from the celestial spaces. 



On my first consideration of this matter I was inclined to believe that the 

 curiously pitted or honeycombed character of the lunar surface, which becomes 

 more and more clear as the magnifying power of the telescope is increased or 

 the seeing more favorable, might possibly be explained by the supposition that 

 the cavities were produced by the in-fall of meteorites of considerable size. 



