THE AID OF THE ACHROMATIC FRINGES. 



119 



in their branches or V-shaped. At first, furthermore, the M effect is stronger 

 when this mass is on the right (R) than when on the left (L) of m; but eventu- 

 ally (after 100 minutes) the two effects are practically identical. Even if the 

 needle were not quite symmetric, this effect is probably extraneous, since it 



m 



vanishes. After 125 minutes the needle was allowed to swing for a time 

 exceeding its period. In the first branch some foreign influence shows itself 

 at a. The second branch is very uniform and precipitate, but the double 

 amplitude obtained is too large. 



91. Reversal after equal time intervals. In contrast with this the graph 

 figures 1 1 8 and 119 show the motion when the reversal of M takes place every 

 10 minutes for no minutes and finally, after an interval exceeding the period 

 of the needle. The curve shows the behavior of the needle during about 5 

 hours, and starts from the equilibrium position. The needle had been rehung. 

 The turning-points of M are indicated by small circles and the excess of motion 

 thereafter is due to inertia. During the first 100 minutes the motion proceeds 

 with remarkable regularity. Both elongations are gradually approaching a 

 maximum, from which, if the logarithmic decrement X and the rotation coeffi- 

 cient b were known, the gravitational force could be computed. There is, 

 nevertheless, a persistent drift of the needle to position much beyond its 

 equilibrium position. This might be due to lack of symmetry in the location 

 of m relatively to the two positions of M, but is probably the result of extraneous 

 causes, and after 100 minutes shows a marked and sudden increase. During 

 the remaining full periods this effect vanishes again and the double ampli- 

 tudes run through successive values of 0.22, 0.18, 0.21, 0.15, 0.14, and 0.15 cm., 

 the latter being nearly symmetrical to the equilibrium position. At the end 

 of a period there is always some irregular oscillation of fluttering, but obviously 



