OCEANIC DISCOVERIES. 659 



from my own observations in the Pacific, that under certain 

 local relations, as for instance during the season of the con- 

 stant 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 con- 

 sideration of an important cosmical event, that with the ex- 

 ception 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 physicists 

 of the present day are still occupied. On the remarkable chart 

 of America, appended to the edition of the geography of Pto- 

 lemy, published at Rome in 1508, we find the magnetic pole 

 marked as an insular mountain, north of Gruentlant (Green- 

 land), which is represented as a part of Asia. Martin Cortez 

 in t\\e Breve Compendia de laSphera (1545), and Livio Sanuto 

 in the Geographia di Tolomeo (1588), place it further to the 

 south. The latter writer entertained a prejudice, 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 mira- 

 culoso stupendo effetto") 



Attention was directed at the close of the fifteenth and 

 the beginning of the sixteenth century, in reference to 

 the distribution of heat and meteorology, to the decrease of 

 heat with the increase of western longitude* (the curvature 

 of the isothermal lines) ; to the law of rotation of the winds, 

 generalized by Lord Bacon ; f to the decrease of humidity 



* 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 land 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 Almirante cap 66). The early conjecture of 

 Columbus regarding the curvature of the isothermal lines in the 

 Atlantic Ocean was well founded, if limited to the extra-tropical (tem- 

 perate and cold) zones. 



f An observation of Columbus, (Vida del Almirante, cap. 55; 

 Examen crit., t. iv. p. 253; and see also vol. i. p. 322 J 

 2 u 2 



