58 TERRESTRIAL MAGNETISM. 



existence of four such lines. As a ship's reckoning requires 

 an exact knowledge of the distance passed over, as well as 

 of the exact direction of the course (given by the angle mea- 

 sured by the corrected compass), the introduction of the 

 use of the " log/' imperfect as this kind of measurement is 

 even at the present day, was yet an important epoch in the 

 history of navigation. I think I have proved, contrary to 

 the prevailing opinion hitherto, that the first certain evi- 

 dence ( 57 ) of the employment of the log (la cadena de la 

 popa, la corredera) is to be found in Antonio Pigafetta's 

 ship's journal in Magellan's voyage, in an entry appertaining 

 to the month of January, 1521. Columbus, Juan de la 

 Cosa, Sebastian Cabot, and Vasco de Gama, were all un- 

 acquainted with the log and its applications. They 

 estimated the ship's rate of movement by the eye alone, 

 and judged of the distance passed over by " hour glasses/' 

 i. e. by the running-out of sand in the " ampolletas." At 

 length, in 1576, in addition to the horizontal declination 

 from the geographical north, which had been so long ex- 

 clusively regarded, the second element, the inclination, or 

 dip of the needle below the horizontal line, came also to 

 be measured. Robert Norman, the first who determined 

 it, did so in London with an instrument of his own inven- 

 tion, and with no inconsiderable degree of accuracy. Fully 

 two centuries more elapsed before any attempt was made to 

 measure the third element, viz. the intensity of the earth's 

 magnetic force. 



A man whom Galileo admired, although Bacon altogether 

 overlooked his merits, William Gilbert, brought forward, 

 at the end of the 16th century, the first enlarged and 

 comprehensive view( 58 ) of the earth's magnetism. He first 



