728 PROCEEDINGS OF THE AMERICAN ACADEMY. 



combustible will burn on the outside and the heat be carried off in the 

 products of combustion blown from it ; while if we stop the experiment 

 at any time we will find the interior of the mass remaining still solid 

 and cold. 



Hence in the case of a large iron meteor there is no such thing pos- 

 sible as an instantaneous vaporization of the whole mass. On enter- 

 ing the thin upper air it will condense the air immediately in front of 

 it, increasing its temperature and raising the density of the oxygen to 

 a point to begin the combustion of the outer layers of the moving mass. 

 If the velocity be extremely high, the opposing pressure in front as it 

 passes into lower air may be sufficient to crush the mass into fragments. 

 The smaller pieces formed will now burn and the rate of waste by com- 

 bustion be greatly increased. This process may go on until the whole 

 is consumed and dissipated in the train. But if the velocity is not high 

 enough for this, the mass either does not fracture or the breaking soon 

 ceases and the remaining flight is marked by combustion or fusion and 

 the continual cleansing of the surface of its fused products, as outlined 

 above. If any of the mass has size enough to endure the rapid waste 

 by oxidation, there may at the last remain a greater or less fraction of 

 the original ma,ss, which reaches the earth's surface in the solid and 

 relatively cold condition, embedding itself in the earth or in the sea. 

 The indications then are that a body which has so survived will neither 

 be moving at an excessive velocity as compared with that of a projectile 

 from a high-powered gun, nor be a hot body on striking, except as to a 

 mere film of its outer surface. As such bodies rarely descend vertically 

 they must pass through many miles of comparatively dense air when 

 their course is more or less horizontal or inclined to the vertical. This 

 and their irregular form renders possible a very great retardation ; far 

 exceeding in proportion that experienced by a well proportioned cannon 

 shot in its flight. If crushing or fracturing took place, angular frag- 

 ments would be split off, very irregular in form, making very poor pro- 

 jectiles, retarded rapidly. The crushing of the mass by opposing air 

 pressure can be compared to the result of the explosion of a charge of 

 nitroglycerine upon a mass of iron, the effect produced being that due 

 to a high gas pressure suddenly formed. But the pressure which would 

 be needed to fracture an approximately round and solid mass of cold 

 nickel iron is very great, so great in fact that it is probable that only 

 those meteors moving relatively to the earth at the very highest veloci- 

 ties would experience it. In fact it may be doubted that such fracturing 

 often occurs with a solid iron meteorite of round form, on account of 

 the somewhat gradual retardation, smaller in the upper air and increas- 

 ing as the body reaches denser air, thus gradually robbing the body of 



