METEORITES, METEORS, AND SHOOTING-STARS. 739 



these meteoroids 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 meteoroids was received in a few hours, 

 do not appear to have been followed by noticeable increase of heat in 

 the air. 



Again, the meteoroids 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 mat- 

 ter 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 

 Professor Oppolzer. But his result would require for each meteoroid 

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

 the supposed power of such small bodies, bodies so scattered as these 

 are even in the densest streams, to break up the comets or other heav- 

 enly bodies, and also their power by intercepting the sun's rays to 

 affect our weather, must in the absence of direct proof to the contrary 

 be regarded as insignificant. 



So, too, their effect in producing geologic changes by adding to the 

 earth's strata, has without doubt been very much over-estimated. Dur- 

 ing a million of years, at the present rate of say fifteen millions of me- 

 teors per day, there comes into the air about one shooting-star or 

 meteor for each square foot of the earth's surface. To assume a suf- 

 ficient abundance of meteors in ages past to accomplish any of these 

 purposes is, to say the least, to reason from hypothetical and not from 

 known causes. The same may be said of the suggestion that the 

 mountains of the moon are due to the impact of meteorites. Enor- 

 mously large meteoroids in ages past must be arbitrarily assumed, 

 and, in addition, a very peculiar plastic condition of the lunar sub- 

 stance 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 mountain-walls, two and three and four miles high, and yet 

 the mountain-walls not sink down again. 



The known visible meteors are not large enough nor numerous 

 enough to do the various kinds of work which I have named. May 

 we not assume that an enormous number of exceedingly small meteor- 

 oids are floating in space, are falling into the sun, are coming into our 

 air, are swept up by the moon ? May we not assume that some of 

 these various forms of work, which can not be done by meteoroids 

 large enough for us to see them as they enter the air, are done by this 

 finer impalpable cosmic dust ? Yes, we make such an assumption. 

 There exist, no doubt, multitudes of these minute particles traveling 



