374 Professor Forbes on Terrestrial Magnetic Intensity, 



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 obser- 

 vations were put in the form of equations of condition for 

 the alpine series, exactly as in the case of intensity ; x re- 

 presenting the variation of dip in minutes for l f of latitude N. 

 increasing; y the variation for 1' of longitude E. increasing; 

 z the variation for 100 feet of height. Geneva is taken for 

 the standard of comparison as before ; 8 A ' representing the 

 correction of the dip at that place. 



Table XL 



Equations of Condition for Dip. 



Geneva 0'* -f O'y + z + S A' = 0' 



Cologny .... 0* + 2y + 3 z + 3 A' = — 8 



Breven —16* + 41 y + 71 z + S A' = — 11 



Chamouni .. —17* + 43 y + 21 z + & A' = — 5 



Jardin —17* + 50 y + 77 z + I A' = — 7 



Aoste — 26**+ 7ly + 6z+&A' = — 18 



St. Bernard.. —20* + 61 y + 68 z + d A' = — 10 



Martigny .... — 6x + 56y + 3« + 3A'= — 26f 



Bex 3* + 52 y + z + $A'= — 5 



Interlaken .. 30* +103 y + 6 z + 3 A' = 17 



Interlaken .. 30* + 103 y +62 + & A' = 20 



Hospital .... 24* + 135 y + 36 z + & A' = 21 



St.Gothard.. 22* + 145 # + 58 z + 3 A' = 5 



Locarno.... — 2* +159 y — 6 z + I A' = —5 



Pfeffers 47* +200 y + 17 z + 5 A' = 4 



49. The method of least squares gives us from these equa- 

 tions the following values of the unknown quantities: — 



x = 0'*543 y = - 0''028 z = 0''080 8 A' = — 3'-4. 

 As already stated, I consider these numbers (particularly z $ 

 which gives an increase of dip of 1' for 1250 feet of ascent) as 

 considerably uncertain. 



50. 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 will 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° l'*6, and the dip would increase 10' for an increase of 

 18'-4 of latitude. 



* The coefficient ought to have been 28. 



f This observation is certainly erroneous, and should have been dis- 

 carded. 



