14 FieLp CoLtumpian MusrEumM—GEoLocy, VOL, 1. 
They are hence often called thumb marks (Phillips Co., 350, Kesen 
257, Floyd Co. 154)— = pee Plate Vb iovar, 
In the iron meteorites these are usually of greater size and depth 
and occasionally perforate the mass (Cafion Diablo, 146). See Plate 
1G isd vie eeatc 
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 4t 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 1° C. by moving 
through air at a velocity of 175 feet per second, and that a velocity 
of 372 feet per second gave arise 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 360,000°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 a piece 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- 
