MAGNETIC SURVEY OF PENNSYLVJ N I .\ M 



Representation of the Total Force. 

 From the expressions 



X 3.900 - 0.1934 dL + 0.0134 dMcos Z, 

 / = 73°.25 + 0.912 dL— 0.0690 dM cm L, 

 we have to deduce the total force <j> = X m I 



In the expression for X, dL = iat. — 41°.34 and </.)/_ long. — 77 . 15. 

 In the expression for /, dL = lat. — 41 .32 and '/J/ - long. — 77 .39 

 We have in 



Long. 81°.00 X= 4.200 ) ..._ 



Lat. 39.97 / = 71 sjs ) * ~ lo4 ' 



Long. 77°.50 X= 3.600 1 ,„._ 



Lat. 12.89 /=74°.676j' > 



Long. 74.00 X= 4.200 ) ,„ , Q 



Lat. 39.60 / = 71°.861 J 9 



Assuming; in the expression for the total force, 



<p = <p n + / + xdL + ydM cos L, 



dL and f/J/as in the expression for Xwe find : — 



$> = 13.55 + 0.0451 dL — 0.00682 //J/ cos /.. 



The lines of equal total force of 13.45, 13.5, 13.55, and 13.6 pass through the 

 following positions: — 



L3.45 . . . Long. 81° Long. 77 : .5 



Lat. 39° 31' Lat. 3!)° 07' 



13.50 . . . Long. 81° Long. 77°.5 Long. 74° 



Lat. 40° 37' Lat. 40° 13' Lat. 39° 40' 



13.55 . . . Long. 81° Long. 77°.5 Long. 74° 



'.at. 41° 43' Lat. 41° 19' Lat. 40° 55' 



13.60 . . . Long. 81° Long. 77°.5 Long. 74° 



Lat. 42° 49' Lat. 42° 25' Lat. 42° 01' 



The observed and computed values of <p at the stations where the bar and 

 cylinder were employed, compare as follows : — 



n 



