THE MAGNETIC POLES. 59 



Netherlanders, British, Spaniards, and French, which had 

 been so widely extended by more perfect methods of determ- 

 ining the direction and length of the ship's course, increased 

 the knowledge of those lines of no variation which, as I have 

 already remarked. Father Acosta had endeavored to reduce 

 into a system.* Cornelius Van Schouten indicated, in 1616, 

 points lying in the midst of the Pacific and southeast of the 

 Marquesas Islands in which the variation was null. Even noAy 

 there lies in this region a singular, closed system of isogonic 

 lines, in which every group of the internal concentric curves 

 indicates a smaller amount of variation.t The emulation 

 which was exhibited in trying to find methods for determin- 

 ing longitudes, not only by means of the variation, but also 

 by the inclination (which, when it was observed under a 

 cloudy, starless sky, aere caliginoso^X was said by Wright to 

 be "worth much gold"), led to the multiplication of instru- 

 ments for magnetic observations, while it tended, at the same 

 time, to increase the activity of the observers. The Jesuit 

 Cabeus of Ferrara, Ridley, Lieutaud (1668), and Henry Bond 

 (1676), distinguished themselves in this manner. Indeed, 

 the contest between the latter and Beckborrow, together 

 with Acosta's view that there were four lines of no variation 

 which divided the entire surface of the earth, may very prob- 

 ably have had some influence on the theory advanced in 168.3 

 by Halley, of four magnetic poles or points of convergence. 



Halley is identified with an important epoch in the history 

 of terrestrial magnetism. He assumed that there was in 

 each hemisphere a magnetic pole of greater and lesser intens- 

 ity, consequently four points with 90° incHnation of the 

 needle, precisely as we now find among the four points of 

 greatest intensity an analogous inequality in the maximum 

 of intensity for each hemisphere, that is to say, in the rapid- 

 ity of the oscillations of the needle in the direction of the 

 magnetic meridian. The pole of greatest intensity was situ- 



* Historia Natural de las Indias, lib. i., cap. 17. * 



t Cosmos, vol. i., p. 181. 



X In the very careful obsen'ations of inclination which I made on the 

 Pacific, I demonstrated the conditions under which an acquaintance 

 with the amount of the inclination may be of important practical util- 

 ity in the determination of the latitude during the prevalence, on the 

 coasts of Peru, of the Garua, when both the sun and stars are obscured 

 (Cosmos, vol. i., p. 180). The Jesuit Cabeus, author of the Philoso- 

 phia Magnetica (in qua nova quaedam pyxis explicatur, <\n&i poll eleva- 

 tionem ubique demonstrat), drew attention to this fact during the first 

 half of the 1 7th ccnturv. 



