88 



DISPLACEMENT INTERFEROMETRY BY 



these are actual changes in the inclination of the pier as a result of non- 

 uniform heating ; if the viscous deformations of apparatus were accentuated 

 with rise of temperature, the two curves should not be in opposition and there 

 does not seem to be any means by which mere thermal expansion could pro- 

 duce this result. 



^ After February 10 the effect of the temperature of the laboratory to be 

 inferred in contrasting the two curves is no longer apparent. In its place, 

 however, is an interesting indication of the effect of external meteorological 

 conditions on the inclination of the pier. Thus, while the curve has in a 

 general way been falling toward the time of the intense cold spell culmina- 



Jaau 17 Z\ % 



ting in February 5, the marked thaw following soon thereafter is accompanied 

 by a rise of curve as far as February 18 and later. Then, with a second cold 

 spell, the curve falls again to February 24 and in turn rises with the next 

 thaw as far as February 27. It seems hardly probable that so marked a gen- 

 eral behavior can be a mere coincidence, and I have regarded it probable 

 that the hill on which the laboratory stands is undergoing similar inclinations. 

 In conclusion, therefore, one may regard the general trend of the curve 

 upward as following from some yield within the parts of the apparatus. On 

 this is superimposed the effect of the warping of the pier from internal 

 causes, (chiefly change of temperature) and the change of inclination of the 

 laboratory as a whole, due to external climatic conditions. 



51. Observations continued. The apparatus was now taken down for re- 

 pairs and some modifications. A brace was added at t" (fig. 57), support- 

 ing the end of the interferometer platform, an addition to which I was at first 

 averse; but if thermal expansion is equal throughout the iron framework, 

 it should not produce discrepancies. The new data (figs. 62 to 65) contain 

 observations between February and August, a period of about 5 months. 

 They are constructed in the same way as the preceding (fig. 61) and places 

 where readjustment was needed are marked a. Moreover, the adjustment 



