ACOUSTICS AND GRAVITATION. 



127 



the window and the torsion of the fiber had to be changed. Subsequently, 

 repulsion in the morning and attraction in the afternoon were the rule, as 

 shown in figures 162 a, 6, and c, N and S denoting a north and south posi- 

 tion of M. The mean excursions, Ay 3 , were, for instance, as follows: 



Sept. 5. Sept. 6. Sept. 7. 



n h Ayj= 2.00 n h Ay, = 1.67 n h Ay t = 2.92 

 I* 1-77 i h 0.48 i b 0.45 



5 h +2.73 4 h +1.14 5 h +2.14 etc. 



This apparatus, with a narrow (1.3 cm., inside) metallic case, and an east- 

 west position of needle, is thus singularly and immediately responsive to tem- 

 perature in a way opposite to No. /. No. Ill was therefore again abandoned 

 as useless for plenum work. 



/4 15 tf 1T ' 18 ' 19 



Apparatus II in a brick niche, narrowly and completely surrounded by 

 brick-work, without outside exposure, and kept in the dark, nevertheless soon 

 began to show a remarkable drift; yet it rarely behaved abnormally. On 

 September 5, when this drift was exceptionally great, apparatus 77 and 7 are 

 out of accord; but on a number of the following days the march of results is 

 similar to apparatus 7, in spite of the difference of environment. The zigzags 

 in 77 are much subdued, as one would anticipate from the secondary reflections 

 received. In fact, No. 7 confronts an exteriorly illuminated wall in the morning 

 and 777 in the afternoon. Thus the former is high in the morning and 777 in 

 the afternoon, indicating the rotation of the sun. It is none the less difficult 

 to understand why 777 registers repulsions in the morning (fig. 162). Heat 

 seems to pass more rapidly through the thin metal case than, so far as temper- 

 ature effect is concerned, it can enter the massive ball M. 



Finally, after observing that the needle, if stuck to the sides, could always be 

 freed by exhausting the case, experiments with 777 were resumed at low pres- 



