EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



The curve as a whole represents the contortions of the pier, probably while 

 it was being gradually dried out from its moist condition of the summer by 

 the steam heat radiating from the steam-pipes in the room. This was partic- 

 ularly apparent toward the end of the curve, at a time when the room for 

 incidental reasons happened to be excessively hot. But a continual increase 

 of a is apparent, showing that the structure as a whole was gradually tipping 

 in one direction. Blasting operations were in progress in a tunnel underneath 

 the hill, but it is not probable, judging from later results, that these affect 



1 



a 



7 



A 



/ 



8 



40 



14 16 



FIG. 5. 



24 



the readings of the horizontal pendulum in the lapse of time. It is impossible 

 to come to definite conclusions at present, but it is not out of the question 

 that actual seasonal changes in the hill itself have also been recorded. Thus 

 the occurrence of a gale always produces a marked temporary effect. I have 

 not, however, thought it worth while to compare the graph of fig. 5 with other 

 graphs (temperature, etc.) which were simultaneously taken, as such work, 

 to be trustworthy, must be done in this laboratory in the summer months, 

 and it will then be advantageous to use the interferometer, as shown below. 



4. New apparatus, without float. To mount the symmetrical form of 

 pendulum described below, an iron scaffolding was installed (in the absence 

 of a suitable pier), erected on the cement foundation layer of the physical 

 laboratory. The truss sustaining the optical parts is shown in perspective 

 in fig. 6 and the horizontal pendulum independent of this, i.e., free from it, 

 in fig. 7. In practice the apparatus, fig. 6, surrounded fig. 7. The feet BA, 

 BC, B'A', B'C' of the framework are bolted to the firm layer of cement at 

 AC and A'C and carry the horizontal rods GH, which with DI make a paral- 

 lelogram. FGE is the rod for mounting the optical parts, secured by the 

 braces DG and DEcaFD in a vertical plane. Lateral braces EH, El, FH, 



