214 OBSERVATIONS OF THE TERRESTRIAL 



land ; and for our present purpose it will be enough to seek the 

 mean direction of the lines, and the mean rate of increase in the 

 direction perpendicular to them. We must therefore employ in 

 the preceding formulas the values of /, h, and 8, corresponding to 

 the mean point of the island, or the point whose latitude and lon- 

 gitude are 53 25' and 7 55',* and for which therefore 



Now it has been already found that 



se w = - -0001633, a? (8) - + -3239, 

 y (k ) = + -0003748, y (8) = - -4950 ; 



and substituting these values in the formulas 



8 - 8 = (jU - fJLo) COS X(j) - (X - Xo)y(S), 



h - h = (ju - juo) cos X#(A) - (X - X ) y(A), 

 we find 



8 - 8 = 21'-4, h-h Q = - -0113. 

 Consequently, 



8 = 7124'-4, A = -9282,/= 1-0295. 



We have now the numerical values of all the quantities which 

 enter the formulas 



P = Jr l x(h) + tan 8 sin l'a?( 8 ), 

 Q = h~ l y( h ) + tan 8 sin ry(s) ; 

 and we find on substitution, 



P = + -0001042, Q = - -0000242. 

 Introducing these values in (V.) and (VI.), 



tan w = - -2322, w = - 13 4', t = -0001102. 

 These results, however, are not entitled to much confidence. 

 An attentive consideration of the formulas (IV.) and (V.) will 

 show that the direction of the resultant isodynamic lines will vary 



* This point corresponds, almost exactly, to the town of Athlonc. 



