404 PROCEEDINGS OF THE NATIONAL MUSEUM vol.90 



An airplane propeller used for a wind tunnel makes the character- 

 istic airplane sound when at full speed. But this is due to the driv- 

 ing of the propeller tips at a velocity close to that of sound, since the 

 electric motor driving it is silent. 



The most conspicuous difference between the thunder sound and an 

 airplane sound is that the latter starts low and rises as the plane 

 draws nearer, since the plane's speed is substantially less than that of 

 sound. But thunder starts with the loudest noise, since it is made 

 by something that is going a great deal faster than sound. 



An unstreamlined body traveling at a speed less than that of sound 

 makes a hissing noise. 



From these facts we may conclude that when the meteor reached 

 Chicora its speed was approximately that of sound, or about 1,000 

 feet a second. Its speed when it struck the atmosphere was probably 

 about 45,000 feet a second. Gravity alone would give it 36,900, to 

 which its velocity of approach must be added vectorially. At this 

 speed its kinetic energy would be: 



% il^F2= 3 1.4X10'' foot-pounds per pound. 



Plate 58 shows the way in which the kinetic energy is absorbed by 

 the air. As the meteor's velocity is far above that of sound, the air 

 in front of it is very highly compressed and is lieated by the compres- 

 sion to white heat, probably to well over 2,000° C. Then a sound 

 wave spreads out from it, like the v/aves from the bow of a boat. 

 And this sound wave may be white hot near the meteor. As before 

 mentioned, this sound wave accounts for part of the energy, 

 which is neglected in the present computation. It also accounts for 

 the thunderclap heard bj^ observers south of Chicora. 



Between the sound wave and the meteor the intensely compressed 

 hot air rushes back into the vacuum behind the meteor and forms 

 a turbulent wake. Except for its incandescence, this wliole appear- 

 ance is similar to that observed in a spark photograph of a bullet in 

 flight.2 



But heat from the air is transferred to the meteor, causing its surface 

 to melt. The molten matter is carried back into the turbulence, where 

 it is atomized to form smoke and its heat given up to the air. An iron 

 meteor loses weight through this melting. But a stony meteor, 

 through at least a part of its course, does not merely melt. It spalls. 

 Pieces of the surface are broken off by rapid expansion and are carried 

 back into the turbulence, where they act as independent small meteors 

 until finally disintegrated and turned into smoke or slowed to the 

 point where further disintegration ceases. The latter is what hap- 

 pened to the two small pieces that were found. 



' See National Bureau of Standards Scientific Taper No. oOS. 



