2g PROFESSOR FORBES'S EXPERIMENTS ON 



48. A careful review of these Observations, compared to those of the usual 

 dipping needles, gives, I think, a favourable impression of the powers of a small 

 instrument. The obsen^ations were put in the form of equations of condition for 

 the alpine series, exactly as in the case of intensity ; x representing the variation 

 of dip in minutes for 1' of latitude N. increasing ; y the variation for 1' of longi- 

 tude E. increasing ; ;:: the variation for 100 feet of height. Geneva is taken for 

 the standard of comparison as before ; S a' representing the correction of the dip 

 at that place. 



Table XL 

 Equations of Condition for Dip. 



Geneva, O'a; + 0> + « + 5 a' = 0' 



Cologny, Ox -\- 2y -'r 5z -{■ Sa'=:— 8 



Breven, . . . . , . . . — 16x + 41 3/ + 71 « + S a' = — 11 



Chamouni, —17a; + 43y -f- 21 « + S a' = — 6 



Jardln, — 17* + SO j + 77 « + S a' = _ 7 



Aoste, — 26 a;* + 71 y + 6 « + S A' = — 18 



St Bernard, . . . • . . — 20 x + 61 j^ + 68 » + J a' = — 10 



Martigny, — 6t + 663^ + Sz -)- S a' =: — 26t 



Bex, Sx + 52y + s + 5 a' = — 6 



Interlaken, 30* + 103.V + 6 « + S A' = 17 



Interlaken, 30x + 103t/ + 6 z + 5 a' = 20 



Hospital, 24 X + 135 1/ + 36 » + J A' = 21 



StGothard, 22x + 146i/ + 68 s + S a' = fi 



Locarao, — 2x + \6Sy — 6s + Sa' = — 6 



Pfeffers, 47 x + 200y + 17 « + 2 a' = 4 



49. The method of least squares gives us from these equations the following 

 values of the unknown quantities : — 



X = 0'.543 ?/ = — 0'.028 z = O'.OBOa a' = — 3.'4. 



As already stated, I consider these numbers (particularly ;:;, which gives an 

 increase of dip of 1' for 1 250 feet of ascent) as considerably uncertain. 



48. If the variation of y for 1' of longitude, be increased in the ratio of the 

 length of 1' of latitude to 1' of longitude (as in Art. 32), it will become = — 0'.039, 

 and the direction of the isoclinal line to the East of North wUl be 



Arc whose tang. = -^ = 85° 53' 



Hence the lines of equal dip would appear to approach nearer to the parallels of 

 latitude than the lines of equal horizontal intensity (Art. 32). The corrected dip 

 at Geneva would be 65° 1'.6, and the dip would increase 10' for an increase of 

 18'.4 of latitude.— See Plate I. 



* The coefficient ought to have been 28. 



t This observation is certainly erroneous, and should have been discarded. 



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