VOL. XVII.] PHILOSOPHICAL TRANSACTIONS. 475 



then give a reasonable account of the four tnagnetical poles, as also of the 

 changes of the needle's variations. 



The period of this motion being wonderfully great, and there being hardly a 

 century since these variations have been duly observed, it will be very hard to 

 bring this hypothesis to a calculus, especially since, though the variations in- 

 crease and decrease regularly in the same place, yet in different places, at no 

 great distance, there are found such casual changes of it, as can nowise be ac- 

 counted for by a regular hypothesis; as depending on the unequal and irregu- 

 lar distribution of the niagnetical matter within the substance of the external 

 shell or coat of the earth, which deflects the needle from the position it would 

 acquire from the effect of the general magnetism of the whole. Of this, the 

 variations at London and Paris afford a notable instance; for the needle has been 

 constantly about 1°^ more easterly at Paris than at London ; though it be cer- 

 tain that, according to the general effect, the difference ought to be the con- 

 trary way. Notwithstanding which, the variations in both places change alike. 



Hence, and from some other of the like nature, I conclude, that the two 

 poles of the external globe are fixed in the earth, and that if the needle were 

 wholly governed by them, the variations would be always the same, with some 

 little irregularities on the account just now mentioned; but the internal sphere, 

 having such a gradual translation of its poles, influences the needle, and directs 

 it variously, according to the result of the attractive or directive power of each 

 pole; and consequently there must be a period of the revolution of this internal 

 ball, after which the variations will return again as before. But if it shall in 

 future ages be observed otherwise, we must then conclude, that there are more 

 of these internal spheres, and more magnetical poles than four, which at pre- 

 sent we have not a sufficient number of observations to determine, and parti- 

 cularly in that vast Mar del Zur, which occupies so great a part of the whole 

 surface of the earth. 



If then two of the poles be fixed, and two moveable, it remains to ascertain, 

 which they are that keep their place ; and though I could wish we had the ex- 

 perience of another century of years to found our conclusions upon, yet I think 

 we may safely determine, that our European north pole, supposed to be near 

 the meridian of the Land's End, and about 7° from it, is that which is moveable 

 of the two northern poles, and which has chiefly influenced the variations in 

 these j)arts of the world: for in Hudson's Bay, which is under the direction of 

 the American pole, the change is not observed to be near so fast as in these 

 parts of Europe, though that pole be much farther removed from the axis. 



As to the south poles, from the like observation of the slow decrease of the 

 variation on the coast of Java, and naar the meridian of the Asian pole, I take 



VOL. III. 3 P 



