ACOUSTICS AND GRAVITATION. 



137 



abundance of drift in the last three days. Figure 170 gives the successive 

 mean double deflections Ay, for a. m. and p. m. These deflections are three 

 to four times too large on the average. 



The straw shaft in apparatus No. V was now withdrawn and a needle with a 

 slender glass shaft substituted. Its dimensions were: stem-length 22 cm.; 

 diameter o.i cm. ; weight 0.326 ; each shot 0.620 gram; total weight with mirror 

 and hanger 1.573 grams. The readings made in a plenum of air are given in 

 figure 171, p. 138. They are much more regular and there is less drift than in 

 the straw-shaft experiments, so that some advantage has been gained, but 

 not nearly enough. The double deflections given in 

 table 6 and figure 170 show a curiously gradual 

 approach to the correct value of Ay, but this is 

 probably accidental. Even the final deflections are 

 about twice too large on the average. 



The experiments with the old apparatus II, also 

 in its former location behind the pier, shut in on 

 all sides but one by heavy brick walls within less than a meter, are shown for 

 the straw-shaft needle in figure 172. They are very irregular and the drift 

 is excessive. On September 21 (R and F refer to rear and front positions of 

 M) there is an inversion. The data for Ay in figure 173 are equally erratic. 



This needle was then replaced by one with a glass shaft, 22 cm. long, o.i cm. 

 in diameter, and stem-weight 0.380 gram; the weight of each shot was 0.600 

 gram and the total weight of the needle 1.585 grams. The new readings given 

 in figure 174 are even worse than the preceding. The values of Ay in table 



6 are equally lawless with enormous inversions (F and R exchanged), mean- 

 ing that repulsions are in excess. 



It is extremely difficult to surmise why this apparatus, in what would be 

 regarded an ideal location, in the dark and virtually constant temperature, 

 should behave in this way; moreover, with the filamentary stem even at 

 a disadvantage as compared with the straw shaft. Electrical charges in 

 a damp basement room are hardly possible. It is more probable that the 

 heavy walls are interchanging heat reservoirs and that the apparatus is in 

 the midst of the radiation. 



Tested with the brass cylinder, JV= 0.0498, the period of the fiber was 



7 = 6.13 seconds. The other quantities in the equation were M= 1590 grams, 

 w = o.6oo grams, 7^ = 4.8 cm., L = 3io cm., I n cm., so that 



7 = io~ 8 9-26Ay 



Thus Ay should have been about 7 mm. Apart from drift, these low values 

 were never reached, even with the straw shaft. Ay, moreover, was always 

 markedly larger in the afternoon. In No. IV they were usually larger in 

 the morning. 



104. Filamentary needle. I thought it desirable to carry the slenderness of 

 needle one step further by making the stem out of the lightest wire possible. 



