THE MOON’S FACE. 
257 
in one night is not great, it has been computed that no less 
than 400 millions are captured by the earth in the course of 
twenty-four hours. So minute are they in general that their 
ashes do not contribute to the earth’s surface an appreciable 
layer of dust; but a few have such size that they are not 
completely consumed in traversing the atmosphere and fall 
to the earth as aerolites weighing grains, ounces, pounds, or 
even tons. For the most part they strike the atmosphere 
with a velocity far higher than could be induced b} 7 the 
earth’s attraction, and we must believe that they are speed¬ 
ing through space in all directions in numbers that defy the 
imagination. They must collide with all planetary bodies 
in numbers depending chiefly on the area of surface exposed, 
and the moon, of course, receives its share.* 
As the moon either is without atmosphere or has one of 
extreme tenuity, the mechanical effect of this bombardment 
may be important, for the average velocity of the meteors is 
from fifty to one hundred times as great as that with which 
the swiftest ball leaves the cannon, and the energy of a pro¬ 
jectile is measured by the square of its velocity. Neverthe¬ 
less it-is incredible that even the largest meteors of which 
we have direct knowledge should produce scars comparable 
in magnitude with even the smallest of the visible lunar 
craters. Recognizing this difficulty, advocates of mete¬ 
oric theories have assumed that at some earlier period the 
meteors encountered by our solar system were of greater 
size than now, and as no evidence has been found that the 
earth was subjected to a similar attack, there is assigned to 
the lunar bombardment an epoch more remote than all the 
periods of geologic history, any similar scars produced on 
* I have discovered no published statement of meteoric theories more 
than twenty years old, but the idea is older and various obscure allusions 
indicate that it was earlier in print. Proctor makes a meteoric suggestion 
in 1873 (The Moon, p. 346), and advocates it in 1878 (Belgravia, vol. 36, p. 
153). A meteoric theory is said to be contained in Die Physiognomie des 
Mondes, by “Asterios,” Nordlingen, 1879. A. Meydenbauer advances an¬ 
other in Sirius, for February, 1882, and he includes bodies other than 
cosmic. 
35—Bull. Phil. Soc., Wash., Vol. 12. 
