140 DISPLACEMENT INTERFEROMETRY- 



where A is the decrement of pressure and Afl the corresponding increment 

 of the velocity of air (from rest) at constant energy. If a is the sectional area 

 of the shot and/ the radiant force, K its value in terms of the contemporaneous 

 gravitation force, it follows further that Ap =//a, and, therefore, 



apR? 



For a plenum of air (p = o.ooi3 and a = o.i6 cm 8 .) this comes out as (Aa) 2 = 

 KXo.o2i. Thus, if K = $, Aa is about 3 mm. per second and A^ = 6Xio~ 5 

 dyne per cm. 2 



In the experiments in vacua, however, K has decreased to 0.3 and p to about 

 0.001293/76 = 0.000017. Thus (Afl^-KX 0.02 1X76 or Az> = o.7 cm. /sec., 

 while Ap is but io~ 6 X4-i dynes /cm 2 . That currents of 7 mm. per second should 

 be possible is difficult to conceive ; but the momentum (vp) carried for the 



7Xi 



present case and the plenum is in the ratio of - - =0.03, which accounts 



3X76 

 for the advantage of the vacuum. 



