44 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



Between the oscillations proper, which are as a rule sharply marked, both 

 at the maxima and the minima, there are regions of relative constancy of 

 inclination. Thus between April 9 and 2 1 the inclination is nearly constant 

 and about 0.6"; between May 17 and 28 there is a slow ascent from about 



4.1" to 3.6"; between June i and 12 a slow descent from 3.0" to 



3.4"; between June 16 and 25, relative constancy at about 2.8", etc. 

 Regarding the observations as a whole, it is not impossible that there may 

 have been some slow yielding (quiescent frictional forces probably yield vis- 

 cously) of the mechanical and the optical parts of the apparatus. The main 

 features of the diagram, however, are due to the pier itself, or the pendulum, 

 in responding to actual forces. On these the former errors may have been 



-28 



-30 



-M . . 



cAflie *8 20 22 



S 



FIG. 26. 



superimposed. Certain large drops on May 15 remain unexplained. The 

 decline after April 3 is large and slow, so that it could be observed daring its 

 occurrence. This may therefore have been actual and not due to displace- 

 ments of the pendulum resulting from subterranean shocks. 



The observations were continued into June and July, with the expectation 

 that when the basement room was no longer heated, the variation of a would 

 practically vanish; but this is not at all the case, as the play of a ii June and 

 July scarcely differs from the average run of values during the winter months. 



From the long-range point of view, inclination, a, decreases from about 

 April i to about May 19, after which it increases intermittently again, recov- 

 ering on June 29 and again on July 27 (not shown), about one-half the total 

 decrement. It would not be difficult to arrange the minima in semi-monthly 

 periods, if any reason for such large variations of a could be assigned. They 

 are of course enormously above anything to be anticipated from tidal influ- 



