169 



sum, and it is only about the sources of the Aire and Ribble, 

 that they are numerous enough to afford a safe average. 



Gearstones 



Horton 



Penyghent 



Settle 



Litton 



Skipton 



Shipley 



These numbers are so nearly balanced as to indicate that the 

 principal local peculiarity in the district belongs to the Vale 

 of York and the Eastern hills, — these contrasted regions 

 having contrary actions on the needle. These actions, whatever 

 they are, die out to the northward, southward and westward, 

 and it becomes an interesting question, whether the curvatures 

 of the Isoclinals be due to the amazing abundance of carbonate, 

 and protoxide, of iron in the hilly district, or to some other 

 mineral quality or structural arrangements of the rocks. 



Dotted lines are drawn on the map to represent the curva- 

 tures of the isoclinals obtained by this discussion. I shall only 

 remark, in confirmation of the general conclusions here arrived 

 at, that magnetic and diamagnetic effects on a suspended 

 needle ought probably to be looked for as a simple consequence 

 of the superposition of strata and their pressure on one 

 another. It seems within the range of possibility to discover 

 by means of the magnet the position assumed by strata far 

 below the surface, and thus to arrive at important knowledge 

 attainable by no other process. 



On Magnetic Intensity/. 



The first observations which I made on this subject had for 

 their object to determine the mean direction of the Isodynamic 

 lines, and the rate of the change of magnetic force in the 



x2 



