Manchester' Memoirs, Vol. xlvi. (1902), No. 10. 7 



gyroscope might under some circumstances be actually 

 used as a compass. But the most interesting of these 

 analogies is seen in its resemblance to the variation of the 

 magnetic dip with the latitude. Thus the maximum dip 

 is located at the magnetic poles of the earth and 

 diminishes towards the magnetic equator, where it is nil, 

 before changing its direction in the southern hemisphere 

 in a manner similar to the variable motion of the 

 pendulum and the gyroscope with the latitude, and the 

 change of the direction of rotation south of the terrestrial 

 equator. 



Just as the pendulum and the gyroscope furnish the 

 most decisive proofs of the diurnal rotation of the earth on 

 its axis that have yet been adduced, so the secular variation 

 of the magnetic elements, as demonstrated by the magnet- 

 arium, are equally decisive proofs of the rotation of the 

 internal parts of the terrestrial globe in accordance with 

 Halley's theory and as confirmed by my own experiments. 

 Hence the gyroscope and the magnetarium are instru- 

 ments complementary to each other for demonstrating 

 the rotations of the exterior and interior parts of the earth 

 respectively. 



Notwithstanding that two centuries have elapsed since 

 the Royal Society published Halley's paper on the motion 

 of the internal parts of the earth as the cause of the mag- 

 netic variation, his theory has since made far less progress 

 than the heliocentric system of astronomy after it was 

 revealed to the world by Copernicus.* In the light of 

 modern knowledge, there is little room for doubt that the 

 slow progress made in this department of astronomical 

 physics is due to the survival of the simian idea of the 

 absolute immobility of the earth, and of the diurnal rota- 

 tion of the celestial bodies. This idea, from the persistency 



* De Revolutionibus Orbium Ccekstium, Lib. I, Cap. lo, p. 9. 1543. 



