42 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



It is also advisable to adjust the plate-glass grating in both cases for compar- 

 ison. In figs. 23, 24, 25, 26 an example is given of these observations in the 

 usual way. Data between January 15 and 24 (fig. 23, A) and March 3 and 6 

 (preceding fig. 23, B] exhibit the behavior of an oil damper with the ruled 

 grating. In the observations after March 14, running as far as July 14, 1914, 

 the ruled grating was replaced by the film grating. Inasmuch as no essential 

 change was made at the steel horizontal pendulum, the constants may be 

 taken to be the same as above, viz, M = i ,2 50 grams ; h = 80 cm. ; R = 1 1 1 cm. ; 

 7=18.48 sec.; 1 = 85 cm.; v> = o.oio8 rad.= 0.62. Thus a=io- 6 X48.6 AN 

 rad. = ioA]V", nearly. 



FIG. 23. 



24. Inferences. The curves in fig. 23, A, B, are independent so far as zero 

 of measurement is concerned, but they already exhibit a tendency to decline 

 in the direction of a decrease of a. This was pronounced in January and also 

 in March. It is not a regular decrease, so that the cause can hardly be, or at 

 least not wholly be, sought in the yield of the parts of the apparatus; for in 

 such a case there would be no recovery (increase of a) , a feature which is often 

 marked. The continuous observations (i.e., with the same uninterrupted zero) 

 are given in figs. 24, 25, 26. The same scale is used throughout, but on April 

 22 and May 15 it was necessary to displace the graph in order to accommo- 

 date the observations on the sheet. The amount of displacement is shown. 

 Here also there is a gradual and continuous decrease of the values of a. Begin- 

 ning on April i with a. about 2", the observations pass through a succession 

 of oscillations to the lowest value of a recorded, about 4.2", on May 18. 

 After this there is intermittent partial recovery, so that on June 28 a has 



