90 DISPLACEMENT INTERFEROMETRY. 



This is at once an evidence of the discrepant effect, direct and indirect, of the 

 heating of the laboratory. In a general way, moreover, rise of internal tem- 

 perature is more apt to be associated with a fall of the a curve, though the 

 similarity is far from consistent. In June and July the continuous general 

 rise of temperature associates itself with a rise of the a curve. Thus the 

 temperature relations, which undoubtedly exist, are very complicated, as 

 is to be expected. During March and April the a values fluctuate between 

 a = 0.6" and a= 1.3", but in May the curve rises and remains permanently 

 above a=i.o". In fact, this trend toward large a values may be said to 

 begin in the middle of April. A comparison with the external temperatures 

 (degrees Fahrenheit) leads to no consistent results. The effect of the thaw 

 about March 9 is apparently well-marked, but the pocket of the a curve 

 may here also be attributed to the ascent of the temperature curve. Subse- 

 quent thaws are not indicated. There are a number of rain-pockets in the. 

 a curve, but they do not bear a definite relation to the amount of precipitation. 

 Rains in summer also usually involve changes of temperature. After the 

 period of comparative quiescence of the a values in May, June, and July the 

 fall recorded after July 24 is peculiar. Nothing local was detected to account 

 for it and it is temporary. 



So far as the observations were made to test the availability of the appa- 

 ratus, they may be regarded as quite satisfactory, but it is obvious they can 

 be used as evidence only during the summer months in the absence of internal 

 heating and that marked temperature changes are menacing under all cir- 

 cumstances. I had hoped that this would not be the case. The conclusions 

 already drawn above thus hold in the sequel. It is my purpose to install 

 this sensitive apparatus under fit surroundings at some future opportunity. 

 It should, then, contribute substantially to geophysical investigation. 



