yf. W. A. Norton 



223 



cold poles, is doubtless not strictly true ; nor have they probably 

 the precise form of any curve disposed symmetrically around p, 

 and therefore the point where the dip is 90° ought not to be in 

 perfect coincidence with the pole of greatest cold. In fact it is 

 conceivable that the distribution of temperature may be such that 

 the needle will no where have the dip of 90°. This would seem 

 to be the case at the Asiatic pole. 



Agreeably to the theory which has been propounded, the mag- 

 netic equator is the line traced through all the points at which 

 the difference of temperature on the north and south is equal to 

 zero ; for at all such places the vertical intensity and therefore 

 the dip would be equal to zero. Now it may be presumed that 

 the places where the difference of temperature is equal to zero, 

 are the warmest points on the different meridians : and, in fact, 

 according to Captain Duperrey, these are precisely the points 

 which lie on the magnetic equator. 



Pole of Maximum Intensity. 



According to Sabine's Chart of Isodynamic Lines, the Ameri- 

 can pole of maximum magnetic intensity is situated in longitude 

 90° to 96°, and latitude 50° to 55°. Professor Locke, of Cin- 

 cinnati, made in the year 1838, &c, a series of observations, 

 which led to the more accurate determination of this point. The 

 result to which he arrived, is that the pole of maximum intensity 

 is situated about in long. 90°. and latitude 47i° ; or nearly on 

 the same meridian with the pole of greatest dip, and about 20° 

 south of it. Let us attempt to determine the situation of this 

 P°mt from theoretical considerations. 



In the first place let it be observed that the horizontal intensity 

 decreases from the equator northward, while the vertical intensity 

 ^creases ; the one varying with the temperature and the other 

 w »th the difference of temperature. It follows from this that the 

 P°le of maximum intensity will be situated on the meridian on 

 *hich the greatest difference of temperature obtains, provided 

 this difference also occurs at a lower latitude than the greatest 

 difference upon any other meridian ; for the maximum vertical 

 density win then be associated with a greater horizontal mten- 

 Sl ty than is the greatest vertical intensity upon any other raerid- 

 lan - The presumption is that the greatest difference of temper- 

 at «re occurs on the meridian which contains the pole of greatest 

 Col< i, since there is a greater variation of temperature from the 

 e( luator to the latitude of the cold pole (70° to 75°), on this than 

 0tl .any other meridian. We may naturally expect also, that the 

 Po.mt of maximum variation will occur farther to the south, upon 



.1 . * *A*M~*.l 



h >s meridian. 



temperatur 



temperature 



