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PHYSICS: THOMPSON, HICKMAN. RIFFOLT Proc. N. A. S. 
which are hurled through a medium to those of practical ballistic design 
or dimensions. Coupled with the spark method of obtaining instantaneous 
photographs of the projectile and the waves connected with its flight, 
it is possible to examine the condition of the medium and the effect o 
change in body shape for a great range of conditions. 
FIG. 6 
The energy given up by a body in motion is dissipated in at least three 
ways — through skin friction, gas compression/ and production of the 
turbulent region particularly evident in the rear (see spark photographs, 
fig. 5). In order to formulate a theory of retardation it is necessary to 
have quantitative information regarding these factors and their relative 
importance. This information is very difficult to obtain. 
