Magnetism. 33 



ing, as is pretended, all the phenomena of terres 

 trial magnetism in different latitudes ; but when 

 applied to recently ascertained facts, their in- 

 accuracy is at once detected. There is reason 

 to hope that the cloud which has long hung over 

 this department of science will speedily be dis- 

 pelled. 



Hitherto the effect of magnetic attraction has 

 only been stated in very general terms, and no 

 attempt has been made to estimate the quantity 

 of 'that effect under different circumstances. 



Mr. Barlow, of the Royal Military Academy, 

 was the first who undertook a regular series of 

 experiments with a view to this determination, 

 and he soon found that there were three distinct 

 conditions to be attended to, viz. the position of 

 the needle and compass, with respect to the 

 attracting body, the mass, or rather the surface 

 of that body, and the distance at which the ac- 

 tion took place. With respect to position, he 

 discovered that a plane may be conceived to be 

 drawn through the centre of attraction of any 

 mass of iron, inclining from north to south at an 

 angle equal to the complement of the dip, in 

 which plane the iron has no effect on the needle ; 

 that is, while the pivot of the compass is found in 

 this plane, the needle will have its true magnetic 

 bearing the same as if no iron were in its vicinity. 

 He also discovered the law of deviation out of 

 that circle, showing it to depend upon the angle 

 which the compass formed with the above plane, 



