EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 45 



ences. So the maxima could be placed at March 17, April , May 9, June 

 J 5f July March 29, April 27, May 29, June 28, July 27, in which the monthly 

 periods are at least marked. But here again any adequate cause for such 

 behavior has not been found, nor in any case has it been possible to separate 

 the true from the adventitious tilting record. 



As the pendulum was a thin steel tube and the direction north-south, 

 one might infer changes of the earth's horizontal intensity. It is hardly 

 probable, however, since whatever magnetization was present was induced by 

 the earth, that forces of the required intensity could be present. The 

 mechanical force at the grating for the displacement A7V would be, roughly, 

 F = 43AA r . Since i" of arc of a was about loAA/", the mechanical force in 

 question is thus 4.3 dynes per second of a. Such forces are not liable to be 

 of magnetic origin. 



Finally, if we compare the run of air temperatures given after May 28, for 

 instance (the thermostat sheets were not accurate enough) , though there is no 

 detailed resemblance in the two graphs, some relation is none the less apparent. 

 Thus the fall of temperature up to June 10 and its rise through a maximum 

 on June 14, to fall again to June 22, is followed by the pendulum graph with a 

 lag. So also the next temperature maximum on June 27 is followed by a 

 pendulum maximum. This lagging of the inclination of the massive pier is 

 precisely what one should expect if the observed oscillations are of thermal 

 origin. It would seem that the parts of the pier exposed to the light expand 

 and contract on the more equally temperatured colder parts, as an axis, as it 

 were. The result would be a pendulum mechanism, very similar to the trian- 

 gular bracket which I have discussed above, 13, and which is peculiarly sensi- 

 tive to the elongation of its parts. The expansion of any side of a triangle 

 produces relatively marked tilting of the axis when the instrument of detection 

 is a horizontal pendulum. 



Taking the observations as a whole, there seems thus to be very little 

 opportunity in the case of an ordinary massive pier of conducting observations, 

 when fixity of inclination within i" of arc is in question, even for brief periods 

 of time. Thus even after June 28, in case of the observed pier, there are 

 changes of a amounting to 2" of arc in ten days, and 0.2" of arc per day must 

 be looked upon as no unusual occurrence. 



25. Improved aluminum pendulum. Observations. The outstanding ques- 

 tion bearing on the above observations was the possibility of a magnetic influ- 

 ence in case of the horizontal pendulum made of steel tubing, the pendulum 

 being otherwise admirable because of its relative strength. A new pendulum, 

 built entirely of aluminum tubing, with the exception of the brass clutch and 

 the vertical hard-steel bearings for the pivots, was therefore installed. The 

 aluminum tubes were screwed firmly together, the large triangle having the 

 following dimensions and constants: Mass of pendulum, 554 grams; mass of 

 grating holder and leveler, 456 grams; mass of (single-plate) film grating, 114 

 grams; mass of damper, 60 grams. This brings the total weight up to 1,124 



