86 DISPLACEMENT INTERFEROMETRY. 



The interesting feature of the curve is this : When observations are made 

 daily, implying, therefore, daily torsional interference with the wire, the vis- 

 cous yield is relatively great. When the wire is left quiet without interference, 

 as during July and a part of August, the yield is much less pronounced. It is 

 not improbable that the molecular disturbances due to the slight inadver- 

 tent twisting (vibration) during observation may account for this result. 

 The motion of the undisturbed wire may even be retrograde, as in August. 

 If this inference is correct, it points seriously to the instability of structure 

 of a permanently twisted wire, even when the twist is not excessive. 



