ACOUSTICS AND GRAVITATION. 



115 



The needle lay somewhat obliquely across the case, the front end being 

 nearer the front window and the rear end nearer the rear, to the effect that an 

 increased potency (high temperature) of these windows would deflect the 

 needle toward larger figures. This is the case in figure 145, where the ordinates 

 are the scale-readings in centimeters and the abscissas change in steps of three 

 minutes, roughly. The amount of the exhaustion o to 10 cm., 10 to 20 cm., 

 etc., is inscribed on the curve, as to the larger number, remembering that 

 the steps except the last (70 to 72, 72 to 74) are each 10 cm. At the end are 

 two cases of influx with a reversal of result. 



On account of the slow and cautious exhaustions made, the correspond- 

 ing temperature decrement of the air within the case must have been very 

 small ; and yet the excursions of the needle due to the attraction of the nearer 

 and relatively hotter glass window is at first very marked, for the deflection 

 produced by M=i.s kg., at 5 cm. should be but 3 cm., roughly, in the same 

 scale. The amount diminishes, however, until between 60 and 70 there is no 

 effect, or an equilibrium of radiant forces. Thereafter, 60 to 70, 70 to 72, 72 to 

 74, the effect changes sign and these repulsions increase at a relatively very 

 large rate. The effect is reversed on compression. 



The experiment was now varied, merely by the changing of the environment ; 

 i. e., the ball M = 1,600 grams was placed within 5 cm. of the needle, in such a 

 way that its gravitation attraction or its radiant force for a plenum and hot 

 ball M would deflect the needle toward smaller numbers; i. e., to act against 

 the effect of the window or case. The results are given in figure 146 and are 

 totally different from the preceding, as there is a double inversion for each 

 exhaustion. At first the case (c) acts as in figure 145; thereafter the effect of 

 the more massive ball M (6) supervenes and finally the needle returns to its 

 equilibrium position. The null effect here occurs between 50 and 60 cm. 

 The inversion, 60 to 70, 70 to 74, is as marked as before. The circles in figure 

 145 are higher than in figure 146, owing to the gravitational attraction of M. 



_ UC. JV* ' 



60 80 



Finally, figures 147 and 148, the torsion-head of the needle was slightly 

 rotated so as to place it in a more symmetrical position to the walls. The re- 

 sults are again noteworthy. In the first place, the phenomenon, figure 147, as 

 a whole, has been reversed through partaking of the same general character. 

 Furthermore the interval of absence of radiant force has been lowered to be- 

 tween 40 and 50 cm. At the same time the radiometer effect at the end has 



