ACOUSTICS AND GRAVITATION. 113 



15, or 6 times the normal value. The ball was next cooled slightly by sub- 

 merging it in tap-water, with the results of figures 140 d and 141 d (enlarged). 

 There is a drop of y = 3 s. p. It was now again submerged, dried, and replaced, 

 with the result of a further drop in deflection of 3.1. On alternating the posi- 

 tion of M from the front (F) to the rear (R), the effect of gravity was found to 

 be just exceeded by the repulsive radiant force from the cooled ball ; for the 

 F deflections are normally the larger. 



Finally, the case itself was covered (except at the mirror) with a thin 

 sheet of metal closely fitting it, and the effects of alternating the position of 

 M were tested through this metallic sheath, with the results of figure 141 e- 

 The excursions of the needle are not only large and irregular, but on the 

 whole a reversal of the gravitational effect. Hence the metallic sheath was 

 again discarded as introducing unnecessary complications and affording no 

 advantages. 



But after long repose (days), the new apparatus functioned quite as well as 

 the old. This may be seen both in figure 142 and others below, or in the 

 three-minute alternation in figure 1 43 . There is little drift and the mean value 

 of the triplets as indicated on the curve is Ay = 0.866, somewhat smaller than 

 in the old apparatus, where M is nearer m. 



It is natural to refer the radiation effect to the reduction of pressure into 

 the energy of motion. The convection between the shot m and heated ball M 

 may be regarded as in excess of that on the other side of the shot, and hence a 

 larger amount of the former pressure is changed to kinetic energy. This would 

 similarly account for the observed reversal in case of a cold ball. There is thus 

 a peculiar radiometer effect in a plenum of air, tending to vanish on exhaustion, 

 to which further attention is given in the next paragraph. 



90. Radiation effect observed on exhaustion. The effect of exhausting the 

 case in small steps of 10 cm., slowly, is to cool the air contained. Hence the 

 result should be like that produced by a hot body on the outside, provided the 

 needle is not quite symmetrically placed with respect to the glass windows of 

 the case and provided the needle end on the side of the hot body is also the 

 nearer. The needle in the present experiments was, accidentally, so hung. 

 Apart from this, the present experiments have nothing to do with the outside 

 environment, if it does not change. They are then merely concerned with the 

 interaction of the needle and the case when the air within is successively and 

 slowly exhausted. In the experiments summarized by figure 144 a, the needle 

 was at rest at 10.7 scale-parts. Exhaustions were then made from o to 30 cm. 

 (lost), 30 to 40 cm., 40 to 50 cm., etc., finally 70 to 74 cm. ; the curve shows the 

 effect produced. After every exhaustion the needle was allowed to return to 

 its zero-point (10.7, see arrows) nearly. The result is very interesting, for the 

 effect of exhaustion changes sign after about 60 or 70 cm. of pressure have been 

 removed. If the effect was like an attraction before, it is like a repulsion after. 

 The curve b, with a somewhat lower zero-point (9.2), shows the same effect for 

 somewhat irregular steps of slow exhaustion, o to 10 cm., 10 to 20, etc., as 



