TERRESTRIAL MAGNETISM. 179 



When the needle, by its sudden disturbance in its horary 

 course, indicates the presence of a magnetic storm, we are 

 still unfortunately ignorant whether the seat of the disturbing 

 cause is to be sought in the Earth itself or in the upper re- 

 gions of the atmosphere. If we regard the Earth as a true 

 magnet, we are obliged, according to the views entertained 

 by Friedrich Gauss (the acute propounder of a general theory 

 of terrestrial magnetism), to ascribe to every portion of the 

 globe measuring one eighth of a cubic meter (or Sy^ths of a 

 French cubic foot) in volume, an average amount of magnet- 

 ism equal to that contained in a magnetic rod of 1 Ib. weight.* 

 If iron and nickel, and probably, also, cobalt (but not chrome, 

 as has long been believed),t are the only substances which 

 become permanently magnetic, and retain polarity from a 

 certain coercive force, the phenomena of Arago's magnetism 

 of rotation and of Faraday's induced currents show, on the 

 other hand, that all telluric substances may possibly be made 

 transitorily magnetic. According to the experiments of the 



I have observed a variation of the magnetic inclination amounting to 

 9 (centesimal division) ; and from Callao to Guayaquil, which differ in 

 latitude by 9 50', a variation of 23-5. (See my Relat. Hist., t. iii., 

 p. G22.) At Guarmey (10 4' south lat.), Huaura (11 3' south lat.), 

 aud Chancay (11 32' south lat.), the inclinations are 6'80, 9, and 

 10-35 of the centesimal division. The determination of position by 

 means of the magnetic inclination has this remarkable feature connected 

 with it, that where the ship's course cuts the isoclinal line almost per- 

 pendicularly, it is the only one that is independent of all determination 

 of time, and, consequently, of observations of the sun or stars. It ig 

 only lately that I discovered, for the first time, that as early as at the 

 close of the sixteenth century, and consequently hardly twenty years 

 after Robert Norman had invented the inclinatorium, William Gilbert, 

 ia his great work De Magnete, proposed to determine the latitude by 

 the iuclination of the magnetic needle. Gilbert (Physiologia Nova dt 

 Magnelc, lib. v., cap. 8, p. 200) commends the method as applicable 

 " aere caliginoso." Edward Wright, in the introduction which he 

 added to his master's great work, describes this proposal as " wortli 

 much gold." As he fell into the same error with Gilbert, of presum 

 ing that the isoclinal lines coincided with the geographical parallel 

 circles, and that the magnetic and geographical equators were identic- 

 al, he did not perceive that the proposed method had only a local and 

 very limited application. 



* Gauss and Weber, Rewltatede* Magnet. Vereint, 1838, $ 31, s. 140. 



t According to Faraday (London and Edinburgh Philosophical Maga- 

 zine, 1836, vol. viii., p. 178), pure cobalt is totally devoid of magnetic 

 power. I know, however, that other celebrated chemists (Hemrich 

 Rose and Wdhler) do not admit this as absolutely certain. If out of 

 two carefully-purified masses of cobalt totally free from nickel, one ap- 

 peal's altogether non-magnetic (in a state of equilibrium), I think it 

 probable that the other owes its magnetic property to a want of puri'.y ; 

 ud this opinion coincides with Faraday's view. 



