14 Field Columbian Museum — Geology, Vol. t. 



They are hence often called thumb marks (Phillips Co., 350, Kesen 

 257, Floyd Co., 154). See Plate V, Fig. 1. 



In the iron meteorites these are usually of greater size and depth 

 and occasionally perforate the mass (Canon Diablo, 146). See Plate 

 III, Fig. 3. 



Similar pittings are observed upon partially burned grains of 

 gunpowder picked up after the firing of the heavy guns at Woolwich, 

 also upon the touch-holes of the cannon and upon masses of steel 

 acted upon by an explosion of dynamite. 



They are due in all these cases to the erosive action of gas re- 

 volving rapidly and moving spirally under high pressure, which bores 

 into a solid mass with which it comes in contact as resistlessly as a 

 gimlet. 



This mechanical action is, moreover, accompanied by a chemi- 

 cal action resulting from the combustible nature of iron at high tem- 

 peratures. 



While at first thought it is difficult to realize how a medium so 

 thin as air can offer resistance to the passage of a meteorite sufficient 

 to fuse its surface, it can be better understood by bearing in mind 

 the fact that air is a fluid made up of molecules as real as those of 

 iron, and physically differing from them only in being more widely 

 separated. A solid body, therefore, in moving through the air, com- 

 presses these particles, and by friction against them generates an 

 amount of heat corresponding to its velocity. Experiments made by 

 Joule and Thomson showed that a wire was warmed i° C. by moving 

 through air at a velocity of 175 feet per second, and that a velocity 

 of 372 feet per second gave a rise in temperature of 5.3 C. Suppos- 

 ing, therefore, that the temperature would continue to increase as 

 the square of the velocity, it can be calculated that a velocity of 20 

 miles per second, which is the average rate at which meteorites strike 

 the atmosphere, would develop a temperature not far from 36o,ooo°C, 

 in a mass of the same character. We may therefore consider a 

 meteorite in its contact with the atmosphere as exposed to a heat 

 capable of melting it as readily as apiece of tallow is melted by being 

 drawn across white hot iron, so that the wonder is, not that it is so 

 easily fused, but that anything is left of it to reach the earth. 



We are thus able also to understand the phenomena of light, 

 of clouds, of smoke and of sounds like thunder or of an explo- 

 sion, which usually accompany the fall of a meteorite. 



The intense heat raises to incandescence the surface of the me- 

 teorite, causing it to glow with a light so powerful as occasionally to 

 be visible at noon-day. The heated stratum of air agglomerates be- 



