Earth's Rocks to Meteorites. 233 



putation shows that the average shooting star ought to have 

 a mass enormously greater than is obtained from the most 

 prolific stone-fall. 



Moreover, if these meteoroides are the source of the solar 

 heat, their direct effect upon the earth's heat by their 

 impact upon our atmosphere ought also to be very great ; 

 whereas the November star-showers, in some of which a 

 month's supply of meteroids was received in a few hours, 

 do not appear to have been followed by noticeable increase 

 of heat in the air. 



Again, the meteoroides do not cause the acceleration of 

 the moon's mean motion. In various ways the meteors do 

 shorten the month as measured by the day. By falling on 

 the earth and on the moon they increase the masses of both, 

 and so make the moon move faster. They check the moon's 

 motion, and so, bringing it nearer to the earth, shorten the 

 month. They load the earth with matter which has no 

 momentum of rotation, and so lengthen the day. The 

 amount of matter that must fall upon the earth, in order to 

 produce in all these ways the observed acceleration of the 

 moon's motion, has been computed by Prof. Oppolzer. But 

 his result would require for each meteroid an enormous 

 mass, one far too great to be accepted as possible. 



The power of such small bodies to break up comets or 

 other heavenly bodies is insignificant, and their effect in 

 producing geologic changes by adding to the earth's strata 

 has been much over-estimated. To assume a sufficient 

 abundancy of meteors in ages past to accomplish any of 

 these purposes, is to reason from hypothetical and not from 

 known causes. 



The same may be said of the suggestion that the moun- 

 tains of the moon are due to the impact of meteorites. 

 Enormously large meteoroides in ages past must be arbitra- 

 rily assumed, and, in addition, a very peculiar plastic con- 

 dition of the lunar substance in order that the impact of a 

 meteoroid can make in the moon depressions ten, or fifty, 

 or a hundred miles in diameter, surrounded by abrupt moun- 

 tain walls two, and three, and four miles high, and yet the 

 mountain walls not sink down again. 



16. 



