THOMSON. — THE FALL OF A METEORITE. 727 



of it reaches ground. The whole energy of the flight will go into the 

 trail or train. The fused iron oxide as soon as formed on its surface 

 is blown off and left behind in the trail which marks the course of the 

 body in the air. If, however, the iron mass or fragment is large, and 

 its velocity insufficient for crushing or further breaking, it may not be 

 entirely burned during its flight and some of it may reach the ground 

 or the sea while still retaining a considerable velocity. It may bury 

 itself in the earth to a depth more or less great. In the case of an iron 

 meteorite there are two sources of heat energy and attendant lumi- 

 nosity, the high compression of the air in front of it and the wastage 

 by combustion of iron in the hot compressed oxygen. The mass is in 

 fact virtually blown upon by a blast oi oxygen in a high state of com- 

 pression. As evidence of the rapidity and effectiveness of such com- 

 bustion reference may be made to the work of the acetylene oxygen 

 blowpipe now used to cut through masses of iron, as in clearing wrecks 

 and the like. The product of the combustion in these cases is mag- 

 netic or black oxide of iron in a fused and perhaps partly vaporous 

 state, and this is removed or blown off" the surface of the iron mass as 

 fast as it is formed. Melted pear-shaped drops have indeed been ob- 

 served as falling out of the track or train left by a meteor in its course 

 through the air. They are probably iron cinder or fused magnetic 

 oxide. It will be understood from this that the energy given out as 

 heat is not delivered to the body of the meteor as such, but is carried 

 off" in the air and oxide layer ripped off" and left behind in the train. 

 It has not been unusual to dig up a meteor seen to fall, and find it 

 stone cold or only slightly warmed. Entering the air from space its 

 temperature wiU naturally be very low, not much above absolute zero. 

 Its flight in the air lasts so short a time that its conductivity for heat, 

 even when it is of solid metal, is not adequate for the instant passage 

 of heat to its interior. It can possess only a thin skin of hot material, 

 a mere film in which the temperature gradient is very steep or abrupt, 

 and this film is constantly blown off" or removed as soon as its tempera- 

 ture of fusion is reached. This temperature is about 1500° C. The 

 case may be illustrated by turning a vigorous blast of heated air upon 

 a piece of ice supported in any way. The ice melts rapidly and the 

 water formed is blown off" as fast as it appears, while what remains is 

 none the less ice to the end of the process. It is even possible that 

 the blast be highly heated so as to boil the water formed without essen- 

 tially changing the result of causing the ice to melt away rapidly but 

 remain while so diminishing a piece of ice to the last. But the ice is 

 not combustible. Let us therefore substitute for it a ball of wood or a 

 mass of combustible and turn thereon a blast of hot oxygen or air. The 



