Terrestrial Magnetic Intensity. 365 



diminishing. Now both these points being best ascertained 

 for No. I., I prefer abiding by its indication. In fact, it ap- 

 pears by Table VII. that the intensity of the flat needle de- 

 creased from August to November (by the Geneva observa- 

 tions) faster than the mean rate of decrease allowed; the con- 

 sequence of which would necessarily be, that the standard in- 

 tensity at Geneva for purposes of comparison would be as- 

 sumed too high, and, as the general order of the observations 

 lay southward and eastward, the apparent increase of intensity 

 in those directions would be smaller than the true, which would 

 give rise to an error of the kind mentioned in Art. 31. The 

 stability of No. I. renders its indications the most certain. The 

 agreement as to the effect of height is very satisfactory, con- 

 sidering the minuteness of the quantity. The source of error 

 just alluded to would scarcely affect this result. The most 

 probable intensity for Geneva will be 1-0776 for No. I.*, and 

 1*0670 for the flat needle. The results are projected in the 

 mapf. 



34. The observations in the Pyrenees lie within smaller 

 compass, and were chiefly conducted with a view to deduce 

 the influence of height. The sources of local error arising 

 from metalliferous deposits are, however, perhaps greater in 

 this case. 



35. Proceeding exactly as before, taking Luz in the valley 

 of Lavedan or Bareges, Hautes Pyrenees, as our point of re- 

 ference, we obtain the following equations of condition from 

 Table VII., which may be arranged exactly as in Table VIII. 

 incorporating the results of both needles. In this case the 

 longitudes being westerly, the variation in longitude must be 

 reckoned the opposite way from that in the former case. 



Table IX. 

 Equations of Condition for the Pyrenean Series. 



Luz(l.) 0'*+ 0'y+ Oz+ai'= -000 



Luz(2.) 0x+ Qy+ Oz+lV = — 001 



Ste Marie 8*— 13y+ 4s+H'= — -004 



Pic du Midi Ax- Qy^2z-\-lV = — 005 



Picde Bergons(l.) — x— 2y+A5 z + Sl' = —-010 



(2.) — x— 2y+45* + 3I' = — -010 



Gavarnie — 8*+ y+21 z+ZV = +-001 



Breche de Roland — 10.T+ 0^+69 z+S I' = --001 



36. Combining these by the method of least squares, we 

 obtain the following values : — 



x — — '000210 



jjrss + -oooioo 



z = — -000053 

 I T s» — -0028. 

 • The intensity varies 01 for 27'5 of latitude. f See p. 375.— Edit. 



