128 DISPLACEMENT INTERFEROMETRY APPLIED TO 



sures throughout. These are marked on the curves, figures 160 and 161. It was 

 inferred that as the air is removed, the effectiveness of convection currents 

 ceases more and more, and therefore the earlier or low-pressure radiant forces 

 should disappear. 



The resumption of the work with No. Ill under partial vacua (marked 

 on the curve) after September 8 soon showed that pressures even as low as 

 35 centimeters were quite inadequate to remove the radiant repulsion specified. 

 With these No. Ill behaved in a way which was exactly the opposite of No. 7. 

 Consequently, after September 14, No. Ill was observed at high vacua only, 

 the pressures ranging from p=i to 2 cm. of mercury. Under these circum- 

 stances the records of No. 7 (observed in plenum) and No. Ill (observed in 

 vacuum) are substantially alike, a result, at first, quite perplexing. 



It follows, therefore, that whereas in case of No. II (wood) the vacuum re- 

 moves a radiant attraction, in case of No. Ill (metal) the vacuum removes a 

 radiant repulsion, the radiant forces being throughout large as compared with 

 gravitation. In fact, even at p < i . 5, the effect of exhaustion has not subsided, 

 for direct tests showed that dy/dp = o.s; i. e., an addition of 5 mm. to y for 

 each mercury centimeter of pressure reduction was still outstanding; twice 

 as much, therefore, for the double amplitude Aj. 



This is a relatively enormous discrepancy. Thus, there must be an inversion 

 of radiant forces with decreasing pressure p, from the repulsions at high pres- 

 sure through zero to the attractions at low pressures or vacua. This complete 

 opposition in the behavior of two apparatus (II, III) in the same position and 

 alike, except as to the wood case of one and the metal case of the other, is 

 difficult to explain. The active needle-end is relatively hot in the metal case 

 and cold in the wood case, or vice versa, under like circumstances, if the simple 

 theories adhered to above are applicable. 



With regard to the metal case 771, it is easy to show, by using a hot body in 

 place of the mass M, that the radiant forces are increasingly repulsive for p < 

 4 cm. and increasingly attractive as pressures are larger. The exact pressure 

 corresponding to radiant equilibrium is a little more difficult to determine. 

 In relation to apparatus 7 and 777, therefore, this implies that in the 

 former case M is warm in the morning (eastern exposure, radiant force 

 attractive) and for apparatus 777, M is relatively warm in the afternoon 

 (southern exposure). Consequently the afternoon repulsive force, operative 

 in apparatus 777 under high exhaustion and warm M, balances the afternoon 

 repulsive force in the plenum apparatus 7, with a relatively cold M. Hence 

 the curves for the two apparatus are ti:e same in kind. In other words, one 

 can thus, in a dark room, demonstrate the rotation of the sun. 



99. Metallic case cooled by efflux. The marked difference in behavior of 

 apparatus 777 (glass and brass) as compared with apparatus 7 and 77 (glass 

 and wood) makes it seem probable that the results of 90 will also be modified 

 in the present experiments; and this is markedly the fact. Unfortunately, in 

 No. 777 the needle hung very near the metallic bottom of the case, so that 



