DISPLACEMENT INTERFEROMETRY 115 



except at the virtually tetrahedral bracket, consisting of horizontal iron rods 

 supporting the interferometer and the iron brace downward from their ends 

 to the pier; for here the oblique part is of iron and the vertical part of brick, 

 and there might be differential expansion. But the interferometer would not 

 be sensitive to this motion, of which I convinced myself by bearing down on the 

 ends of the bracket with the hand. No adequate displacement of fringes 

 resulted. Hence it appears probable that what is observed is the warping 

 of the pier, etc., as a result of the inward progress of the successive isotherms 

 through it, beginning at the part least protected by surrounding walls. At 

 all events, this temperature feature is so serious that a few tenths of a degree 

 centigrade can not be overlooked. It is, then, upon a substratum undergoing 

 continuous warping that any other phenomenon of more relevant interest 

 must be superimposed. This would make their detection and interpretation 

 so different that only under conditions of adequately constant temperature 

 could the extreme sensitiveness realized be made practically available. 



