170 



SCIEN'CE. 



[Vol. VIII., No. 185 



sociation exists, it is true, for the advancement of 

 science, but science may be advanced by rejecting 

 bad hypotheses as well as by framing good ones. 



I begin with a few propositions about which 

 there is now practical unanimity among men of 

 science. Such propositions need only be stated. 

 The numbers that are to be given express quanti- 

 ties that are open to revision and moderate 

 changes. 



1. The luminous meteor tracks are in the upper 

 part of the earth's atmosphere. Few, if any, ap- 

 pear at a height greater than one hundred miles, 

 and few are seen below a height of thirty miles 

 from the earth's surface, except in rare cases 

 where stones and irons fall to the ground. All 

 these meteor tracks are caused by bodies which 

 come into the air from without. 



2. The velocities of the meteors in the air are 

 comparable with that of the earth in its orbit 

 about the sun. It is not easy to determine the 

 exact values of those velocities, yet they may be 

 roughly stated as from fifty to two hundred and 

 fifty times the velocity of sound in the air, or of a 

 cannon ball. 



3. It is a necessary consequence of these veloc- 

 ities that the meteors move about the sun and not 

 about the earth as the controlling body. 



4. There are four comets related to four peri- 

 odic star-showers that come on the dates April 

 20, August 10, November 14, and November 27. 

 The meteoroids which have given us any one of 

 these star-showers constitiite a group, each indi- 

 vidual of which moves in a path which is like that 

 of the corresponding comet. The bodies are, how- 

 ever, now too far from one another to influence 

 appreciably each other's motions. 



5. The ordinary shootuag-stars in their appear- 

 ance and phenomena do not differ essentially from 

 the individuals in star-showers. 



6. The meteorites of different falls differ from 

 one another in their chemical composition, in their 

 mineral forms, and in their tenacity. Yet through 

 all these differences they have peculiar common 

 properties which distinguish them entirely from 

 all terrestrial rocks. 



7. The most delicate researches have failed to 

 detect any trace of organic life in meteorites. 



These propositions have practically universal 

 acceptance among scientific men. We go on to 

 consider others which have been received with 

 hesitation, or in some cases have been denied. 



With a great degree of confidence, we may be- 

 lieve that shooting-stars are solid bodies. As we 

 see them they are discrete bodies, separated even 

 in prolific star-showers by large distances one from 

 another. We see them penetrate the aii- many 

 miles, that is, many hundred times their own 



diameters at the very least. They are sordetimea 

 seen to break in two. They are sometimes seen 

 to glance in the air. There is good reason to be- 

 lieve that they glance before they become visible. 



Now these are not the phenomena which may- 

 be reasonably expected from a mass of gas. In 

 the first place, a spherical mass of matter at the 

 earth's distance from the sun, under no constraint, 

 and having no expansive or cohesive power of its 

 own, must exceed in density air at one-sixth 

 of a millimetre pressure (a density often obtained 

 in the ordinary air pump), or else the sun by his 

 unequal attraction for its parts will scatter it. 

 Can we conceive that a small mass of gas, with no 

 external restraint to resist its elastic form, can 

 maintain so great a density ? 



But suppose that such a mass does exist, and 

 that its largest and smallest dimensions are not 

 greatly unequal ; and suppose further that it im- 

 pinges upon the air with a planetary velocity : could 

 we possibly have as the visible result a shooting- 

 star ? When a solid meteorite comes into the air 

 with a like velocity, its surface is burned or melted 

 away. Iron masses and many of the stones have 

 had burned into them those wonderful pittings 

 or cupules which are well imitated, as M. Dau- 

 bree has shown, by the erosion of the interior of 

 steel cannon by the continuous use of powder 

 under high pressure. They are imitated also by 

 the action of dynamite upon masses of steel near 

 which the dynamite explodes. Such tremendous 

 resistance that mass of gas would have to meet ! 

 The first effect would be to flatten the mass, for it 

 is elastic ; the next to scatter it, for there is no 

 cohesion. We ought to see a flash instead of a 

 long burning streak of light. The mass that 

 causes the shooting-star can hardly be conceived 

 of except as a solid body. 



Again, we may reasonably believe that the 

 bodies that cause the shooting-stars, the large fire- 

 balls, and the stone-producing meteor all belong 

 to one class. They differ in kind of material, in 

 density, in size. But from the faintest shooting- 

 star to the largest stone-meteor, we pass by such 

 small gradations that no clear dividing lines can 

 separate them into classes. See wherein they are 

 alike : — 



1. Each appears as a ball of fire traversing the 

 apparent heavens, just as a single solid but glow- 

 ing or burning mass would do. 



2. Each is seen in the same part of the atmos- 

 phere, and moves through its upper portion. The 

 stones come to the ground, it is true, but the lu- 

 minous portion of their paths generally ends high 

 up in the air. 



3. Each has a velocity which implies au orbit 

 about the sun. 



