220 



Prof. W. A. Norton on Terrestrial Magnetism. 



does not exist, since the magnetic equator crosses the geographi- 

 cal equator very near the coast of Africa. 



We have seen that for the coast of Europe, and of Africa, the 

 excess of the mean temperature at the equator over that at any 

 latitude /, is proportional to 1-cosn/; n having various values 

 at different latitudes, intermediate between 2 and 1-5. This 

 makes the formula for the vertical intensity in western Europe 

 and Africa, 



Ver. intensity 



Y 



cos n(l - 5°) - cos n(l+5) cos D 

 cos n(L - 5°) - cos w(L + 5°) cos d 



It 



This formula gives results too small in the lower latitudes, 

 can however be made to represent the observations by introdu- 



1 + sinnL 

 cmg the factor ^T^^J- This being done, we have 



Ver. intens, — V 



cosw(Z — 5°) 



(H-5 



COS 



1+ 



cos»(L— 5°) — cosn(L-f-5°) cos d l-f-sinn/' '*"' 



Whether the introduction of this factor is due to the fact that 

 the variation of temperature is not uniform from 5° south to 5° 

 north of the given place, as supposed in the investigation of the 

 formula, (becoming necessary here because of the more northern 

 situation of the magnetic equator,) or to some other cause, I have 

 not yet undertaken to determine. It is important to observe that 

 this factor although empirically introduced, is still a function of 

 the differential variation of temperature. For the diminution of 

 temperature in passing from the equator to any latitude I, is pro- 

 portional to 1 - cos nl. The differential of this, or the differen- 



temperature 



In 



j-~-~*„^w. V j Afc ^ WJ.J.J. guv g /K^CC/. ill VJLJI/CH""- 1 ^ 



suits given in the following table, V was at first taken equal to 



1-2445, the observed vertical intensity at Paris, L = 48 

 D = 22°. The vertical intensity for latitude 25°, western coast 

 of Africa, was then calculated by the formula, and found to be 

 1-007. The other determinations were subsequently obtained bjr 

 giving this value (1-007) to V, and taking L = 25°, and D = 2° < 

 So that the calculations were all virtually made from the observ- 

 ed intensity at Paris. The same values of n were used as in the 

 calculations of the horizontal intensities (&>* Table VIII.) 



Table X- 



Fans, 



at. 25 Q , W 



of Africa, 

 Christiania, 

 Edinburgh, 



(jottingen, 



{London, . 



Ver. Intensity. 



computed., [observed. 



I* 007 



i*33o 

 i-35r 



1-223 



1-284 



1*2445 



1*014 

 r353 

 i-34o 



1*257 

 1*282 



Diff. 



-■007 

 -•023 



+•011 



Place. 



Berlin, . 

 Milan, . 

 Marseilles, . 

 Lat. 35°, W. 

 coast of Africa, 



-•o34|Lat.l0°, do. do. 

 +•002 J Lat. 0°, do. do. 



1 Ver. Intensity, 

 'imputed, observed. 



-268 



CO 



par 



differences between the resnlts of computation and obsg 

 both in this and the previous table, cannot but be regard 



