7 3 6 THE POPULAR SCIENCE MONTHLY. 



We see them penetrate the air many miles, that is, many hundred 

 times their own diameters at the very least. They are sometimes 

 seen to break in two. They are sometimes seen to glance in the air. 

 There is good reason to believe that they glance before they become 

 visible. 



Now, these are not the phenomena which may be reasonably ex- 

 pected from a mass of gas. In the first place a spherical mass of mat- 

 ter at the earth's distance from the sun, under no constraint and hav- 

 ing 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 constraint to resist its elastic force 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 impinges 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. Daubree has shown, by the erosion of the interior of steel can- 

 non 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 sepa- 

 rate 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 glowing or burning mass would do. 



2. Each is seen in the same part of the atmosphere, and moves 

 through its upper portion. The stones come to the ground, it is true, 

 but the luminous portion of their paths generally ends high up in 

 the air. 



3. Each has a velocity which implies an orbit about the sun. 



4. The members of each class have apparent motions which imply 

 common relations to the horizon, to the ecliptic, and to the line of the 

 earth's motion. 



