60 .TERRESTRIAL MAGNETISM. 



only that the declination, but also that the inclination, of 

 the magnetic needle might be made available, (such a use 

 of the inclination, with a clouded starless sky, " per acre 

 caliginoso," Wright called " worth much gold,") ( 62 ) led 

 to the construction of many and various magnetic instru- 

 ments, and stimulated the activity of observers. The Jesuit 

 Cabeus of Ferrara, Ridley, Lieutaud in 1668, and Henry 

 Bond in 1676, distinguished themselves in this way. The 

 controversy between Bond and Beckborrow, together with 

 Acosta's view of the existence of four lines of no declination 

 dividing the entire surface of the globe into four parts, 

 may, perhaps, have had some influence on Halley's theory 

 of " four magnetic poles, or points of convergence/' pro- 

 jected as early as 1683. 



Halley's name constitutes an important epoch in the 

 history of terrestrial magnetism. He assumed the existence 

 in each hemisphere (northern and southern) of two mag- 

 netic poles, a stronger and a weaker pole ; and we now find 

 an analogous distribution of the magnetic force in four 

 points of maximum force, two in each hemisphere, one 

 stronger than the other, as shown by the rapidity of the 

 vibrations performed by a needle oscillating in the direction 

 of the magnetic meridian. The strongest of Halley's four 

 poles was placed in 70 S. lat. and 120 E. long, from 

 Greenwich, or almost in the meridian of King George's 

 Sound, in New Holland. ( 63 ) Halley's three voyages in 

 1698, 1699, and 1702, followed the first sketch of his theory, 

 which was founded only on the observations obtained on his 

 voyage to St. Helena seven years before, and on the 

 imperfect variation (declination) observations of Baffin, 

 Hudson, and Cornelius von Schouten. Halley's expeditions 



