20 



PROFESSOR FORBES'S EXPERIMENTS: oN 



Table VIII. — {continued.) 



Jardin, 



Col dcs Fours, 



Aoste, 



St Bernard, 

 10. Martigny, . 



Interlaken, 



Schmadribach, 



Grinddwald (1.) 



Grindelwald (2.) 

 16. Mcyringen, 



Grimsel, 



Miinster, 



Genimi, 



Frutigen, 

 20. Faulhom, . 



Engelberg, . 



Surennes, 



Klus (near Altorf); 



St Gothard, 

 25. Locamo, 



Orta, 



Bellaggio, 



Reichenau, 



Wallenstadt, 

 30. Lucerne, 



Rigi-Culm, 



31. From these thirty-one equations of condition for Needle No. I, and 

 twenty-four for the Flat Needle, we obtain by the method of least squares the 

 following values of the fom- unknown quantities, the calculations having been 

 verified by independent methods. 



X =: Variation of intensity for I' of latitude N.increa.sing, 

 y ■=■ Variation of intensity for 1' of longitude E increasing, . 

 z =: Variation of intensity for 100 English feet of height, 

 51' := Correction applicable to the registered intensity at Geneva, 



32. To deduce from these numbers the lines of equal horizontal intensity, we 

 must remark that the minute of longitude is shorter than the minute of latitude 

 in the ratio of 7^ to 10 nearly, on an average, in the Alps. The variation of y for 

 a geographical mile or minute would therefore be 



For No. I. = + .000076. For " Flat" = -)- 000146 

 And the angle made by the isodynamical lines with the meridian towards the 

 east from north would be 



Arc whose tang. := --^, and arc whose tang. := — ^ 

 76 146 



78° 12' and 73° 52' 



