TERRESTRIAL MAGNETIC INTENSITY. 35 



east, made by the Isodynamical Lines (of horizontal intensity) with the meridian 

 would be, 



Arc whose tan ^j = 71 66'. 



72. If we now assume the values just found for x and y, and compute the 

 horizontal intensities at the preceding stations, we have, I apprehend, very satis- 

 factory evidence of the consistency and accuracy of these observations. 



Horizontal Intensity. 



Place. Observed. Calculated.* Difference. 



Linz, 1.066 1.0669 -f .0009 



Ischl, 1.0785 1.0770 .0016 



Salzburg, 1.073 1.0725 .0005 



Gastein, 1.090 1.0888 .0012 



W. Matrei, 1.0846 1.0881 + .0036 



Inspruck, 1.0775 1.0763 .0012 



Bormio, 1.0875 1.0889 + .0014 



Trent, 1.104 1.1026 .0016 



Laybach, 1.1275 1.1275 .0000 



Vienna, 1.080 1.0802 -f- .0002 



\ 



The mean difference without regard to sign is .0012 ; and were the observation 

 at Windisch Matrei omitted (when a large door-key had inadvertently been left 

 in the pocket), the error would have been very much less indeed. 



73. I have proceeded in a similar way with regard to the small number of 

 observations on dip, which may serve in a general way to indicate the direction 

 of the Isoclinal Lines in the same region. The symbols have the same relative 

 signification as before. Inspruck is here taken for the starting point. 



Equations of Condition for Lines of Equal Dip in the Eastern Alpine Group. 



Linz, 63 x + 172 -f SA' = 26'.7 



Salzburg, 32 x + 98 + 2 A' = 14'.8 



Inspruck, 0#+0-(-SA' = 



Trent, 72 * 19 + 3 A' = 43'.2 



Laybaeh, . . . . 74 x + 262 + I A' = 85'.1 



Vienna, . . . . 57 a + 299 + 2 A' = 2'.3 



From these equations Mr BROUN has also deduced, by the method of least squares, 

 the following values : 



a = Variation of Dip for 1' of Latitude N increasing, + O'.7ll4 

 y = Variation of Dip for 1' of Longitude E increasing, OM3656 

 I A' = Correction applicable to Dip at Inspruck, + 3'.685 



The variation of y for one geographical mile is 0'.20, and the angle towards the 



* From the formula, 



.000386 x + .0000864 y + 1.0889. 



VOI. YV PAPT T v 



