364. 



Professor Forbes's Experiments on 

 Table VIII. — {continued* ) 



ByNo.l, 



Gemini 13*+ 88.y+ 62z+S 1' = -002 



Friitigcn 24*+ 89^+10 z+H' = --003 



-•005 



20 Faulhorn 28.r+lll#+76z + o- 1' 



Engelberg 37 x+ 138^+21 z+S I' = —005 



Surennes 37-r+]443,+64z+B I' as — 005 — -009 



Klus (near Altorf).. 37*+150 < y+ 3z+lV = —004 --010 



St.Gothard 22x+145^+ 58z+S I' = — -005 --009 



25.Locarno — 2x+159y— 6z+%V = -008 -008 



Orta _25.r+135y— 3z + 3I' = '016 



Bellagsrio _12*+187#- 6z+lV = '019 -019 



Reichenau 37*+195^+ 7z+SI' = -000 



Wallenstadt 55*+19ly+ z+M' = — *007 —'008 



30.Lucerne 51 *+130y+ 2z+d I' = — 008 — -013 



Rigi-Culm 51 x+ 140^+46 z+l V = —'014 



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 four un- 

 known quantities, the calculations having been verified by in- 

 dependent methods. 



By "Flat." 

 -•00£ 



-011 



No. I. 



—000364 



x = Variation of intensity for V of latitude N \ 



increasing J 



y = Variation of intensity for 1' of longitude \ 



E increasing J 



z = Variation of intensity for 100 English feet ) 



of height / 



1 1' = Correction applicable to the registered "I i.nnif? 

 Geneva J ' 



+•000055 

 —000033 



Flat. 

 —000505 



—000106 



—000027 



—0040 



intensity at 



32. To deduce from these numbers the lines of equal hori- 

 zontal 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. L = + -000076. For " Flat" = + -000146. 



And the angle made by the isodynamical lines with the meri- 

 dian towards the east from north would be 



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



76 ° 146 



= 78° 12' and 73° 52'. 



33. Of these results I conceive that the former is to be pre- 

 ferred. The discrepancy of the results obtained by No. I. 

 and " Flat" are, I presume, attributable to one or both of two 

 causes, — a progressive change in the magnetic state of the 

 needle somewhat different from what has been allowed for, — 

 and a slight error in the correction for temperature, which, 

 during the period of observation (the autumn), was generally 



