14 Popular summary of Experiments on the 



ments, Mr Christie afterwards found, that not only had 

 different points in the circumference of the same plate differ- 

 ent attracting powers, hut the same point had a different in- 

 fluence according as the plate was made to revolve to the 

 right or left hand. Suppose, for example, any point in the 

 circumference of the plate marked (a,) to be brought opposite 

 to the compass, by turning the plate slowly round to the left 

 hand, and that the entire action of the plate at that time de- 

 flected the needle from its true position 12° ; then, if the 

 plate be again turned on its axis to the right hand, till the 

 same point (a) occupies the same position as before, the de- 

 flection of the needle will not be the same, but be altered, per- 

 haps to 13° or 14°. If it be made to revolve a second, or 

 third, or, indeed, any number of times in the same direction, 

 still the deflection will remain as before, (viz. 13° or 14°, ac- 

 cording to our supposition ;) but if after this it be turned one 

 i-evolution back again to the left, the deflection will become as 

 at first 12°, and this quantity will not be altered by repeating 

 the revolution in this direction. The quantity of deviation 

 stated above is merely assumption, as the actual deviation dif- 

 fers in amount according to the distance of the plate from the 

 needle, and its position with reference to the same. Mr 

 Christie has, by means of a very ingenious machine, been 

 enabled to place the plate in any position, and adopting the 

 views of his friend Mr Barlow, by conceiving an ideal mag- 

 netic sphere to circumscribe the compass, he registers the lati- 

 tude and longitude of the centre of the plate on this sphere 

 with the corresponding deflection in each case, and then sub- 

 mits his experimental results to the test of theoretical investi- 

 gation. We cannot follow the author in this part of his la- 

 bours, but our readers will readily comprehend the following 

 illustrations of the observed effects, (although we are not 

 quite certain that it is the same view which the author him- 

 self has taken of the subject.) 



Let us conceive a plate of iron placed in the magnetic me- 

 ridian. This plate will be polarized by induction from the 

 earth, — that is, the magnetic fluid in the plate will receive a cer- 

 tain polarized direction. If we could now conceive this plate 

 to become hard steel, the magnetic fluid would become fixed, 



