ACOUSTICS AND GRAVITATION. 



125 



with mirror weighed 1.813 grams; of this, the two shots contributed 0.595 

 gram each, the straw shaft 20.9 cm. long (between centers), the remainder, 

 0.303 gram, being in the mirror and hangers. The shaft thus weighed but 

 about half as much as either shot. 



The space within which the needle was to vibrate was only a little over a 

 centimeter; but it usually sufficed here, after the apparatus had been thor- 

 oughly and uniformly cooled. 



Tested in a plenum of air, the apparatus showed itself extraordinarily sen- 

 sitive to temperature, as instanced in the half -hour periods in figure 158 a, 

 where the gravitational attraction is at first ineffective but eventually devel- 

 ops. A part of this is due to the day, as apparatus I also shows it, though not 

 to the same degree. A>' 3 as high as 6 was finally obtained. 



The effect of exhaustion is shown at figure 1586 and there is here an apparent 

 increase of Aj 3 with the pressure p marked on the curves. But while A^ 3 

 increased from 1.6 to 3.1, the corresponding reading of apparatus 7 also 

 increased (for a plenum) from A^ x = 1.2 to 2.1. Thus in these complica- 

 tions there seems to be no margin left for the above effect of exhaustion. 

 The marked irregularities which a variable atmospheric temperature may 

 produce is instanced in figure 158 c, observations obtained with apparatus 

 II on the preceding day (N north and 5 south position of the attracting 

 mass M) . It is not improbable, therefore, that the metallic frame III will 

 in a different way respond to the anomalies which the wood frame showed at 

 reduced pressures. 



Apparatus 777, as stated, was installed on the east-west wall of the pier, 

 in front or facing south, replacing apparatus 77. The latter was now placed 

 in a niche in the rear of the pier, facing a brick wall about i meter to the 

 south. In this position it was completely screened from any direct radiation 

 from the walls of the room having outside exposure. Apparatus 7 on the 

 north-south face of the pier was left in its old position fronting (east) the 

 30-inch wall exposed to the morning sunlight outside, when present. The 

 wall east-west faced by apparatus 777 was always in shadow. It was there- 



