726 PROCEEDINGS OF THE AMERICAN ACADEMY. 



smaller fragments of meteoric iron now found in the rim. Finally, as 

 the masses of material came to rest there would settle back into the 

 excavation a large part of the pulverized material leaving the sheer 

 rock walls as they now are, upturned at various angles even to almost 

 complete inversion in some places. The material which did not fall 

 back to the crater would rest on the upcast walls and slope away upon 

 the surface of the plain in a fairly symmetrical manner. In their flight 

 through the air the iron was burning into oxide ; the heat and debris 

 were left behind in the wake or trail. What reached the earth was 

 comparatively if not actually very cold, retaining in its interior, as it 

 were, something of the cold of space. 



It may be of interest here to present if possible a brief statement of 

 what must happen during the flight of a meteoric mass through our 

 air. In general, the earth's atmosphere acts so effectually as a protec- 

 tive sheath that only a few of the very numerous bodies variously 

 known as shooting stars, aerolites, and meteors ever reach the earth's 

 surface. If the velocity of a body entering from the outside is very high 

 relatively to the earth, thirty or forty miles per second, for example, 

 the crushing strains brought upon it by the air resistance in its path 

 may be great enough to break it into fragments, while the high tem- 

 perature of the compressed air opposing its movement melts or vapor- 

 izes the material composing it. Stony masses, like pieces of rock, 

 would yield to fracture and dissemination more readily than masses of 

 solid iron. This fact may itself account for so large a proportion of 

 the bodies which reach the ground being composed of iron. Rock 

 masses are occasionally found, surviving, perhaps, because of low en- 

 tering velocity. The iron meteors are so strong as to resist enormous 

 crushing strains. The metal iron is, however, so freely combustible 

 that when exposed to a blast of hot, compressed oxygen it is burned 

 into fused oxide very rapidly. So with the iron meteors in their flight. 

 They are virtually blown upon by a highly heated blast of compressed 

 air containing so much oxygen as to cause them to burn like tinder. 

 The survival of a stony meteor, or aerolite, until it reaches the ground 

 must therefore depend upon its entering the air at comparatively low 

 velocities, or upon its being retarded in a long flight through the very 

 thin higher air whereby it loses much of its initial speed before enter- 

 ing the denser air below, while the survival of an iron meteorite de- 

 pends on its velocity being insufficient to develop crushing strains 

 great enough to fracture it into small masses, and upon the size of the 

 meteor itself, or of its fragments, if fractured. A small iron mass at 

 high velocity will burn away so rapidly in the dense oxygen in front 

 of it that the whole mass will be consumed and dissipated before any 



