40 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



In fig. 22 these observations have been inscribed, the ordinates being the 

 inclination of the pier a, in hundredths of a second of arc, very nearly. It 

 will be seen that the inclination increases as a whole from the beginning to 

 the end of the month, the total range lying within something over 2 seconds 

 of arc. The rise is particularly marked and sustained after the i4th, and the 

 difference of inclination between the first and second halves of the month is 

 about i second. 



ZOO 

 180 

 160 

 140 



m 



100 

 80 

 60 

 40 

 10 



\ 



I 



1 



6 



10 H 



FIG. 22. 



# 15 14 15 16 IT 



As the observations were made in an unavoidably steam-heated room, it 

 is probable that the flexure of the pier, etc., due to thermal causes, has been 

 largely operative in modifying the trend of the curve; for on comparing the 

 curve as a whole with the thermostat sheets (not shown) a retarded effect is 

 possibly suggested, such as one would suspect if variations of surface tem- 

 perature should penetrate massive masonry. It would then be possible for 

 the curve to have different heights at the same temperature. Naturally such 

 comparisons are very vague, and it is the range of values of a admissible in 

 the apparatus which is here of paramount interest. Furthermore, as the hill 

 on which the laboratory stands is, at present, being tunneled, so that the 

 building is subject once or twice a day to the tremors resulting from the 

 vigorous blasting underground, adequate conditions for the installation of an 

 apparatus of the present kind are still remote. It is really surprising that 

 interferometer observations could be made, without essential difficulty, under 



