ACOUSTICS AND GRAVITATION. 



129 



great care in exhaustion (slow change of pressure) was necessary; but I do not 

 think this had any influence on the following experiments. These are given 

 in figure 163 and made (as above) by exhausting the case slowly in steps 

 (v) of the pressure-gage, v=o to 15, 15 to 25, etc., cm., as marked on the 

 curves, so that the pressures fall as = 76 to 61, 61 to 51 cm., etc. 



In the series 2, 3, 4, the weight M was removed and the case and needle 

 reciprocate in the absence of M. In series i, the weight M was in front, attract- 

 ing toward larger numbers y, of the scale. Series i and 2 were made after sun- 

 down, at night ; series 3 and 4 in the day-time. The first exhaustion in series i 

 (0.15 cm.) was obviously too fast; but all these series consistently show a new 

 result, different from 90 ; there is no inversion of the immediate effects of ex- 

 haustion, according as they are made at high vacua or at low vacua. The first 

 effect of exhaustion is always an increase of pressure on the hotter side; i. e., the 

 radiometer effect prevails throughout. It seems, however, to pass through a 

 minimum somewhere between 40 and 50 cm., though, as the rate of efflux can 

 not be controlled, this is difficult to assert. In high vacua (about 70 cm.) 

 the effect rapidly increases as it did above. Here there is no difference. 



If we compare series i and 2 another inference may be drawn. The equilib- 

 rium positions from which exhaustions were made are marked by little circles. 

 In series i these rise rapidly, which means that the weight M was at a lower 

 temperature than the needle (and case) and that the radiant forces are largely 

 removed by the exhaustion. Above 70 cm., however, they are again partially 

 restored, and this is also the case in series 3 and 4, where the circles descend 

 after v = 70 cm. Series 2 in the absence of the weight M shows that needle and 

 case were about at the same temperature and that the exhaustion, therefore, 

 has less influence, there being only relatively small radiant forces. In series 

 3 and 4, made in the afternoon, the case was colder than the needle and the 



