ILLUSTRATIONS (Continued) 



5 Summary of differences in isotherm depths . . .15 



6 Example of autocorrelation of successive half-minute 

 readings of isotherm depth values . . . 1? 



7 Summary of autocorrelation of selected isotherm 

 depth values ... 18 



8 Example of power spectrum from successive half-minute 

 readings of isotherm depths . . . 20 



9 Ratio of peak values to background in power spectrum . . . 22 



