130 DISPLACEMENT INTERFEROMETRY. 



strong radiant forces resulting, rapidly dwindle as the exhaustion proceeds. 

 They are never, however, quite removed, as was possible in the case of 

 inversion of radiant forces, above. 



The effect of the successive exhaustions on the motion of the needle washere 

 also very different from observations in the above work. The needle reaches 

 its low positions within two minutes after the exhaustion. It then merely 

 creeps to larger numbers y. I usually waited four or five minutes. By 

 waiting longer, larger numbers would have been reached and no doubt the 

 minima immediately after exhaustion would have been larger. Creeping re- 

 sponds to a developing change of radiant force. 



The effect of exhaustion in removing radiant forces may be used in freeing 

 the needle when it adheres to the windows of the case. As this in No. Ill was 

 very narrow (1.3 cm. inside), the annoyance of needle pushing with its ends 

 the two opposite windows was quite frequent on sunny days, as stated above. 

 It is merely necessary to exhaust the case, without changing the torsion-heads 

 to free the needle, and the apparatus then behaves normally; but if p is variable 

 the attractions of M are not constant, nevertheless showing that a small part 

 of the radiant force persists residually to modify the results. To use III with 

 advantage, it must therefore be kept exhausted to above at least v = 50 cm. 

 or = 25 cm., as is ultimately done in figure 161. 



In the experiments with the wood case, I was under the impression that the 

 radiant forces for low and for high vacua would always be opposite in sign. 

 The experiments of figure 163 show that this is not generally true. It is dif- 

 ficult to ascertain the reasons which govern the sign of the former, and it is 

 probably associated with distributions of temperature in the very narrow 

 metallic case, in a complicated way. Thus by changing the position of the 

 needle, results (of which series 6, fig. 163, is an example) were obtained in 

 which the radiant forces for low vacua are at first positive but become negative 

 at about 30 cm. of exhaustion. Inversion is thus met with even with the 

 metallic case, though the pressure at which radiant equilibrium occurs is not 

 a definite quantity. The fact that so much of the earlier radiant force may be 

 removed by exhaustion may be regarded as a proof of its dependence on 

 convection. The other proof is its gradual development; for it is not present 

 immediately after turning the weight M. 



