OCEANIC DISCOVERIES. 281 



dipping-needle invented in England, in 1576, by Robert Nor 

 man, than Gilbert boasted that, by means of this instrument, 

 he could determine a ship's place in dark, starless nights (aere 

 calignoso).* Immediately after my return to Europe, I show- 

 ed from my own observations in the Pacific that, under cer- 

 tain local relations, as, for instance, during the season of the 

 constant mist (garua) on the coasts of Peru, the latitude might 

 be determined from the magnetic inclination with sufficient 

 accuracy for the purposes of navigation. I have purposely 

 dwelt at length on these individual points, in order to show, 

 in our consideration of an important cosmical event, that, with 

 the exception of measuring the intensity of magnetic force, and 

 the horary variations of the declination, all those questions 

 were broached in the sixteenth century, with which the phys- 

 icists of the present day are still occupied. On the remarka- 

 ble chart of America appended to the edition of the geography 

 of Ptolemy, published at Rome in 1508, we find the magnet- 

 ic pole marked as an insular mountain north of Gruentlant 

 (Greenland), which is represented as a part of Asia. Martin 

 Cortez in the Breve Compendio de la Sphera (1545), and 

 Livio Sanuto in the Geographia di Tolomeo (1588), place it 

 further to the south. The latter writer entertained a preju- 

 dice, which has unfortunately survived to the present time, 

 that " if we were so fortunate as to reach the magnetic pole 

 [il calamitico), we should there experience some miraculous 

 effects {alcun miracuhso stupendo effetto"). 



Attention was directed at the close of the fifteenth and the 

 beginning of the sixteenth century, in reference to the distribu 

 tion of heat and meteorology, to the decrease of heat with the 

 increase of western longitudef (the curvature of the isothermal 

 lines) ; to the law of rotation of the winds, generalized by Lord 



magnetic lines without variation led Halley, by the contests between 

 Henry Bond and Beckborrow, to the theory of four magnetic poles. 



* Gilbert, De Magnete Physiologia nova, lib. v., cap. 8, p. 200. 



t In the temperate and cold zones, this inflection of the isothermal 

 lines is general between the west coast of Europe and the east coast of 

 North America, but within the tropical zone the isothermal lines run 

 almost parallel to the equator ; and in the hasty conclusions into which 

 Columbus was led, no account was taken of the difference between sea 

 and laud climates, or between east and west coasts, or of the influence 

 of latitudes and winds, as, for instance, those blowing over Africa. 

 (Compare the remarkable considerations on climates which are brought 

 together in the Vida del Almiranle, cap. 66.) The early conjecture of 

 Columbus regarding the curvature of the isothermal lines in the Atlan- 

 tic Ocean was well founded, if limited to the extra-tropical (temperate 

 and cold) zones. 



