80 



M A G N E TIC S D R V E Y F P E X N S YL V A X I A . 



The observed and computed values of X by the previous and last hypotheses, 

 compare as follows : — 



For the last hypothesis (straight lines) we find the probable error of an obser- 

 vation and local irregularity from the bar and cylinder vibrations, +0.029; and 

 from the Lloyd needle deflections and dip, ±0.062. For the previous hypothesis, 

 these quantities are respectively, +0.030 and + 0.059, showing but little gain in 

 the representation of tin- observations by the additional term JL dM cos L. 



For the general representation, the probable errors are +0.050 and +0.051. 



