22 



A. A. MICHELSON 



Table III is a table of results for i?o, the return at the time / 

 after releasing the stress/ R^j gives the result of calculation 

 from 



i? = 890(1 -e~">/0- 



Rt gives values calculated from 



i? = 84o(i-e"-^') . 



The differences between calculated and observed values under 

 Ai and A2 show that the former expression is very near the truth, 

 while the latter is entirely inadequate.' 



TABLE III 



While the term involving a permanent set may not have any 

 application to the problem of the earth tides, yet it may not be 

 amiss to draw attention to the fact that in some cases and espe- 

 cially at temperatures approaching the melting-point, this term 

 becomes the most important of all. The temperature coefficient 

 in this case enters in the form 6/T—d, giving as it should perfect 

 fluidity at T, the melting-point. 



In the former article the expression given for this viscous 

 term is ^3 = {Ft)", in which F^C^Pe^'^ and p is stated to be approxi- 

 mately one-half. 



From more recent data the average value of p is .41; and if 

 from the nineteen substances examined four be excluded the average 

 is .35, which makes it much nearer one-third than one-half. 



^It was found by experiment that for stresses not too great the "direct" curve 

 (on applying the stress) and the "return" curve (on releasing) were the same; or 

 <rather if the former is S and the latter R, then S-{-R = Ct. 



