MAGNETISM. 16? 



The line along which the inclination of the needle from the 

 horizon is the same as at York (isoclinal line), runs on the 

 average to the east of north 63 14' ; and there is an interval of 

 1'69 geographical mile between two isoclinal lines, along which 

 the inclination differs one minute of a degree*. Between the 

 south-eastern and north-western extremities of the county, the 

 inclination of the needle varies from 69 55' to 70 45'. 



On the average, as before observed, the isoclinal lines cross 

 the meridian lines in Yorkshire at angles of 63 14* and 116 46', 

 but they are not free from twists and irregularities depend- 

 ing on the nature of the rocks and the masses of land. I find 

 a systematic arrangement of these local deviations from the 

 general direction of the isoclinal lines. They appear to bend 

 southward in crossing the great Vale of York, and to turn up to 

 the northward on the hilly districts which rise to the east and 

 to the west. 



The force with which the magnetic needle is retained in the 

 position which it occupies is found to vary in different parts of 

 the country, and to increase toward the north-west. It is greater 

 at York than at London, in the proportion of T0126 to I'OOOO. 

 The line along which the force is equal (isodynamic line) crosses 

 the meridian in Yorkshire at an angle of 47 37' to the east of 

 north ; and for a change in this force amounting to y^o^ P ar ^ 

 we must pass to the north-west about 112 geographical miles f. 



* These results are from my observations as given by Col. Sabine in the 

 " Magnetic Survey of the British Islands." (Reports of British Associa- 

 tion for 1837.) 



t From my observations as given by Col. Sabine, loc. cit. 



