TERRESTRIAL MAGNETIC INTENSITY. 21 



33. Of these results I conceive that the former is to be preferred. The dis- 

 crepancy 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 au- 

 tumn), was generally diminishing. Now both these points being best ascertained 

 for No. I, I prefer abiding by its indication. In fact, it appears by Table VII. that 

 the intensity of the Flat Needle decreased from August to November (by the Ge- 

 neva observations) faster than the mean rate of decrease aUowed; the conse- 

 quence of which would necessarily be, that the standard intensity at Geneva for 

 purposes of comparison would be assumed 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 indi- 

 cations the most certain. The agreement as to the effect of height is very satis- 

 factory, considering the minuteness of the quantity. The source of error just 

 aUuded to would scarcely affect this result. The most probable intensity for Ge- 

 neva wiU be 1.0776 for No. I,* and 1.0670 for the Flat Needle. The results are 

 projected in the Map, Plate I. 



34. The observations in the F3Ten^es 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 reference, we obtain the following equa- 

 tions of condition from Table VII., which may be arranged exactly as in Table VIII. 

 incorporating the results of hoth needles. In this case the longitudes being wes- 

 terly, the variation in longitude must be reckoned the opposite way fi*om that in 

 the former case. 



Table IX. 

 Equations of Condition for the Pyrenean Series. 



Luz(l.) Car + 0'2/ + Oar + n'= .000 



Luz(2.) Qx ■{■ Oy ■{■ Ox + H' = — .001 



SteMarie, Bw — \^y -|- 4 2; + H' = — .004 



Pic du Midi, 4:00 — Qy + 12 z -\- 21'= — .005 



Pic de Bergons (1.) . . • . . — x — 2y -\- ^ z -\- 21' = — .010 



(2.) . . . . , — X — 2y + ^5z + n'= — .010 



Gavamie, — Bx -\- y-)-21«-|-SI' = -|- .001 



Breche de Roland, — lOar + Oy + 69 a? -f- SI' = — .UOi 



• The intensity varies .01 for 27'5 of latitude. 



