ACOUSTICS AND GRAVITATION. Ill 



always in a consistent phase-difference relative to the alternating gravitational 

 force. In other words, the discrepant force develops in an orderly manner after 

 the weight M is turned. Furthermore, there is in most of the graphs a 

 marked tendency toward uniform motion of the needle between the turning- 

 points or their equivalents. We may thus explain the ratio of total excursions 

 i : 4 : 9, for one, two, and three minute periods, more consistently with the obser- 

 vations as follows : In the first minute after turning there is a mere accommo- 

 dation of force conditions. Hence the one-minute periods nearly always fail to 

 show interpretable results. In the two-minute and three-minute periods the 

 excursion times have thus been only 2-1 minutes and 3-1 minutes, the latter 

 twice the former. The approximate ratios 4: 9 are thus more probably to be 

 taken f or i : 2 or the mere total excursions to be expected from uniform motion. 

 Furthermore, if there is uniform motion, the gravitational attraction is 

 counterbalanced by frictional resistance and the latter may be taken as pro- 

 portional to the speed of the needle. These rates of motion must therefore be 

 looked upon as of much greater interest than the value of the triplets as com- 

 puted above. Moreover, it there is marked drift the value of the triplets 

 will be correspondingly changed, for the force in action is large as compared 

 with gravitation. 



88. New apparatus. Another needle, of the same kind as above, was now 

 installed in a smaller case, capable of exhaustion and useful for other experi- 

 mental work. This consisted (fig. 138 elevation, fig. 139 plan) of a hollow 

 rectangle ww of waxed wood, 1.8 cm. thick, to which glass plates gg' were 

 attached, soft sealing-wax having been 

 melted around their edges, reinforced by 

 steel clips. The needle with the two shots 

 at m, m', 0.56 gram each, and a light mir- 

 ror at n, was supported by the quartz fiber 

 q, from a torsion-head above, carried by the 

 glass tube t. The whole was rendered air- 

 tight by melting soft wax into all crevices 

 and joints. The hooks of the quartz fiber 

 were also cemented together, so that there 

 was nothing loose. The needle was put in 

 place with one of the glass plates off. This was then fastened as stated. It 

 could again be removed by melting the wax at the edges. 



The case was fastened nearly vertically, by aid of the brass rods r, r', held 

 in clamps anchored in the pier. The final leveling and freeing of the needle 

 was accomplished by a screw-pusher p abutting in the pier. The tube t was 

 also held in a clamp from the pier, to prevent excessive tremor. 



The attracting weight M had a separate mounting on the same pier, and by 

 a crank-like mechanism could be smoothly and easily passed between adjust- 

 able stops, from M to M'. The observations were made in the same dark, 



