20 



PROFESSOR FORBES'S EXPERIMENTS ON" 



Table VIII. — {continued.) 



Jardin, 



Col des Fours, 



Aoste, 



St Bernard, 

 10. Martigny, . 



Interlaken, 



Schmadribach, 



Grindelwald (1.) 



Grindelwald (2.) 

 16. Meyringen, 



Grimsel, 



Miinster, 



Gemmi, 



Friitigen, 

 20. Faulhom, 



Engelberg, 



Surennes, 



Klus (near Altorf); 



St Gothard, 

 26. Locarno, 



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 four unknown quantities, the calculations having been 

 verified by independent methods. 



a? =: Variation of intensity for 1' of latitude N. increasing, 

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

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



SI' = 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 T^r to 10 nearly, on an average, in the Alps. The variation ofp for 

 a geographical mile or minute would therefore be 



For No. l. = + .000076. For " Flat" = + 000146 



And the angle made by the isod3Tiamical lines with the meridian towards the 

 east from north would be 



Arc whose tang. := — ^, and arc whose tang. = — -> = 78° 12' and 73° 52' 

 76 14d 

















No. I. 



By" Flat 



11 X 



+ 601/ 



+ 



11 z 



+ 



i] 



^ 



.007 



.009 



21 X 



+ 36y 



+ 



IQz 



+ 



n 



' := 



.012 





2Qx 



+ 712/ 



+ 



Qz 



+ 



n 



' r= 



.020 



.018 



20 X 



+ Qiy 



+ 



68 « 



+ 



S] 



= 



j006 



.000 



Gx 



+ 561/ 



+ 



Zz 



+ 



SI 



' := 



.007 



.004 



QOx 



+ 103 2/ 



+ 



Gz 



+ 



S) 



zzz — 



-.008 



— .009 



IQ X 



+ 1042/ 



+ 



39 « 



+ 



s] 



=: 



•003 



— .002 



26 a? 



+ 113y 



+ 



24 2; 



+ 



S] 



[' = - 



-.001 



.000 



2Qx 



+ 113y 



+ 



21 z 



+ 



S] 



[' ■=. - 



-.004 



— .007 



52 X 



+ 1232/ 



+ 



1 z 



+ 



I] 



z= — 



- .001 





22 ,r 



+ 130 2/ 



+ 



49* 



+ 



S] 



' := — 



-.002 



— .007 



18 a? 



+ 128 2/ 



+ 



29 2: 



+ 



S] 



[' = 



.002 



— .00» 



13 a? 



+ 882/ 



+ 



62 2; 



+ 



S] 



;' =1 



.002 



— .002 



24 a? 



+ 892/ 



+ 



10 2; 



+ 



s 



[' =- 



-.003 





28 a? 



+ 1112/ 



+ 



76 2; 



+ 



s 



;' z= _ 



-.005 



— .011 



37 a? 



+ 138 2/ 



+ 



21 2r 



+ 



I 



[' = - 



-.005 





37 a? 



+ 1442/ 



+ 



64 2; 



+ 



i 



[' =1 - 



-.006 



— .009 



37 a? 



+ 150 2/ 



+ 



3« 



+ 



s 



r := - 



-.004 



— .010 



22 a? 



+ 145 2/ 



+ 



682; 



+ 



s 



[' =1 _ 



-.005 



— .009 



2a? 



+ 159 2/ 



— 



Qz 



+ 



s 



[' = 



.008 



.008 



26 a? 



+ 136 2/ 



— 



3z 



+ 



I] 



=: 



.016 





12 a' 



+ 1872/ 



— 



62; 



+ 



s 



[' =z 



.019 



.019 



37 a? 



+ 195 2/ 



+ 



1z 



+ 



s 



r = 



.000 





56 X 



+ 1912/ 



+ 



z 



+ 



s 



['■ =1 - 



-.007 



— .008 



61 a^ 



+ 130 2/ 



+ 



2z 



+ 



S] 



;' = _ 



-.008 



— .013 



61 a' 



+ 1402/ 



+ 



462; 



+ 



2] 



' =: _ 



-.014 







No. I. 



Flat. 



— 



.000364 



— .000606 



+ 



.000065 



— .000106 



— 



.000033 



— .000027 



+ 



.0016 



— .0040 



