34 PROFESSOR FORBES'S EXPERIMENTS ON 



68. Exactly as in art. 30. of my former paper, I proceeded to calculate the 

 direction of the lines of equal horizontal intensity, and those of equal dip, in the 

 eastern part of the Alps, taking as a basis the observations contained in the pre- 

 ceding tables from Linz to Vienna inclusively. 



69. Assuming Bormio (the most western) as a normal station for intensity, 

 and denoting, as in art. 28, by a and b, the co-ordinates of latitude and longitude, 

 expressed in minutes of a degree for any other station compared to Bormio ; de- 

 noting also by I the observed variation of intensity compared to Bormio, and by 

 S I' the correction applicable to the observed intensity there ; the form of equation 

 to the isodynamic (horizontal intensity) line is, 



a,x + b,y + i I' = I, 



x being the variation due to 1' of latitude, N increasing ; y that due to V of lon- 

 gitude, E increasing. 



70. The following equations of condition were deduced from the last table. 



Equations of Condition for Lines of Equal Horizontal Intensity in the Eastern 



Alpine Group. 



71. These equations having being treated by the method of least squares, 

 the following values of x, y, and <5 V were determined by Mr JOHN A. BROUN : 



x = Variation of Horizontal Intensity for 1' of Latitude N increasing .000386 



y = 1' of Longitude E increasing 4- .0000864 



X I' rz Correction applicable to Intensity at Bormio, . . 4" .00138 



In the former Western Alpine series (art. 31.), we had for the same needle No. 1, 



a = .000364 

 y = 4- .000055 



a satisfactory coincidence. The length of a minute of longitude is .68 of a minute 

 of latitude in the Eastern Alps. Hence the first of the above values of y be- 

 comes for a geographical mile of longitude + .000126, and the angle towards the 



