58 COSMOS. 



steel the preference over soft iron, because the former has the 

 power of more permanently retaining the force imparted to 

 it, and of thus becoming for a longer time a conductor of 

 magnetism. 



In the course of the 17th century, the navigation of the 

 Netherlander^ British, Spaniards and French, which had 

 been so widely extended by more perfect methods of deter- 

 mining 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 endeavoured to reduce 

 into a system. 60 Cornelius van Schouten indicated, in 1616, 

 points lying in the midst of the Pacific and south-east of 

 the Marquesas Islands in which the variation was null. 

 Even now there lies in this region a singular, closed system 

 of isogonic lines, in which every group of the internal con- 

 centric curves indicates a smaller amount of variation. 61 

 The emulation which was exhibited in trying to find methods 

 for determining longitudes, not only by means of the varia- 

 tion, but also by the inclination (which when it was observed 

 under a cloudy starless sky, aere caliginoso? 2 was said by 

 Wright to be "worth much gold") led to the multiplication 

 of instruments 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 probably have had some influence on the theory, ad- 

 vanced in 1683 by Halley, of four magnetic poles or points 

 of convergence. 



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



61 Cosmos, vol. i, p. 175. 



62 In the very careful observations 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 

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

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

 scured (Cosmos, vol. i, p. 173). The Jesuit, Cabeus, author of the Phi' 

 losophia Maynetica (in qua nova qusedam pyxis explicatur, quse poli 



7 elevationem, ubique demonstrat), drew attention to this fact during the 

 first half of the 17th century. 



