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PHYSICS: C. BARUS 
the two curves is marked. After that the gross resemblance is no longer so 
striking, but it nevertheless remains in the details and there is a mere lack of 
equivalence, quantitatively. The question therefore arises where this tem- 
perature discrepancy has its seat. It can hardly be in the interferometer, 
where the parts are of the same metal except in the (virtually) tetrahedral 
bracket, consisting of the rods, E, E', and the brace downward from t" to r 
in the pier. For here one part t"r is of iron and the other from EE' downward 
to r, of brick. There might thus be differential expansion; but the interfer- 
ometer would not be sensitive to this motion, of which I convinced myself by 
bearing down hard at t" with the hands. 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 parts least protected against changes of 
temperature by the surrounding house walls. At all events this temperature 
feature is so serious that a few tenths of a degree centigrade can not be over- 
looked. 
