512 G. Barns — Application of the Displacement 



tunnelled, so that the building is subject once or twice a day 

 to the tremors resulting from the vigorous blasting under- 

 ground, adequate conditions for the installation of an appa- 

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

 that interferometer observations could be made, without 

 essential difficulty, under these circumstances. During an 



Fig. 4. 



Fig. 4. Tilting of the pier in hundredths of a second of arc, showing the 

 range of measurable a. The lower limit of a, 10~ 3 second, would not be 

 visible in the diagram. 



explosion, of course, the ellipses vanish ; to reappear, however, 

 immediately after, sometimes with displacement, such for 

 instance as is indicated by certain of the doubled parts of the 

 curve. The use of the water damper, moreover, which was 

 necessary here, is objectionable, though it has not, probably, 

 introduced any marked error into the observed curve (see also 

 doubled parts). Finally, the use of a steel horizontal pendu- 

 lum with its plane in the magnetic meridian is inadmissible. 

 I have not, therefore, endeavored to interpret the results, but 

 they are given simply as an example of a systematic series of 

 observations, extending over a month. I hope in the summer 

 to resume the work in the absence of the annoyances referred to. 



