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SCIEJ^CE. 



[Vol. VllI , No. 185 



and motion of the air. The smaller meteors visible 

 to the naked eye may be thought of without seri- 

 ous error as being of the size of gravel stones, al- 

 lovs'ing, however, not a little latitude to the mean- 

 ing of the indefinite word 'gravel.' 



These facts about the masses of shooting-stars 

 have imporiant consequences. The meteors, in the 

 first place, are not the fuel of the sun. We can 

 measure and compute within certain limits of error 

 the radiant energy emitted by the sun. The 

 meteoroids large enough to give shooting-stars 

 visible to the naked eye are scattered very irregu- 

 larly through the space which the earth traverses ; 

 but in the mean each is distant two or three hun- 

 dred miles from its near neighbors. If these 

 meteoroids supply the sun's radiant energy, a 

 simple computation shows that the average shoot- 

 ing-star ought to have a mass enormously greater 

 than is obtained from the most prolific stone-fall. 



Moreover, if 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 accelera- 

 tion 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 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 

 heavenly bodies ; and also their power, by inter- 

 cepting the sun's rays, to affect our weather, must, 

 in 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. During a milKon of years, at the 

 present rate of, say, fifteen millions of meteors 

 per day, there comes into the air about one shoot- 

 ing-star or meteor for each square foot of the 

 earth's surface. 



To assume a sufficient 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 tliat the mountains of the moon 

 are due to the impact of meteorites. Enormously 

 large meteoroids in ages past must be arbitrarily 

 assumed, and, in addition, a very peculiar plastic 

 condition 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 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 varioiis kinds of 

 work which I have named. May we not assume 

 that an enormous number of exceedingly small 

 meteoroids 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 results, which cannot be due to me- 

 teoroids large enough for us to see as they enter the 

 air, may be due to this finer impalpable cosmic dust? 

 Yes, we may make such an assumption. There 

 exist, no doubt, multitudes of these minute par- 

 ticles traveUing in space. But science asks not 

 only for a true cause, but a sufficient cause. There 

 must be enough of this matter to do the work 

 assigned to it. At present we have no evidence 

 that the total existing quantity of such fine mate- 

 rial is very large. It is to be hoped that through 

 the collection and examination of meteoric dust 

 we may soon learn something about the amount 

 which our earth receives. Until that shall be 

 learned, we can reason only in general terms. So 

 much matter coming into our atmosphere as these 

 several hypotheses require would, without doubt, 

 make its presence known to us in the appearance 

 of our sunset skies, and in a far greater deposit of 

 meteoric dust than has ever yet been proven. 



A meteoroid origin has been assigned to the 

 light of the solar corona. It is not unreasonable 

 to suppose that the amount of the meteoroid 

 matter should increase toward the sun, and that 

 the illumination of such matter would be much 

 greater near the solar surface. But it is difficult 

 to explain upon such a hypothesis the radial struc- 

 ture, the rifts, and the shape of the curved lines, 

 that are marked features of the corona. These 

 seem to be inconsistent with any conceivable 

 arrangement of meteoroids in the vicinity of the 

 sun. If the meteoroids are arranged at random, 

 there should be a uniform shading away of light 

 as we go from the sun. If the meteoroids are in 

 streams along cometary orbits, all lines bounding 

 the light and shade in the coronal light should 



