WILLIAM GILBERT. 53 



Further, Cardinal de Vitri, who wrote a History of Jerusalem about 

 the year 1200, also describes the magnetised needle as indispensable 

 in navigation. An obscure author, Peter Peregrinus, whose existence 

 was for long considered mythical, and who wrote a letter upon mag- 

 netism reputed to be of a date at the end of the thirteenth century, 

 describes the fact that the north-pointing end or region of one load- 

 stone will attract the south-pointing end or region of another load- 

 stone. Peregrinus's letter was certainly published as a small book of 

 forty-three pages, small quarto, at Augsburg, in 1558. On 14th of 

 September, 1492, Columbus, when about 200 leagues west of the 

 European coast, noticed for the first time the declination of the 

 compass needle from the true north. According to Gilbert, the same 

 discovery was made (in 1498) by Sebastian Cabot. But it was not 

 till the middle of the sixteenth century that accurate measurements 

 were made of the amount of declination in Europe. Robert 

 Norman, a compass-maker in Limehouse, found that the compass 

 pointed 11° 15' to the east of the true north. Borough, Comptroller 

 of the Royal Navy, in 1580 found it to be 11° 19'. The dip of the 

 needle was discovered also by Norman in 1576; and the same fact 

 was independently observed in 1544 by Hartmann, of Nuremberg. 

 Norman constructed a dipping needle, by the aid of which he ascer- 

 tained the angle of dip at London to be 71^ 50'. Another isolated 

 fact was discovered in 1590 by a surgeon of Rimini, named Julius 

 Ccesar, namely, that a vertical bar of iron used as a support on the 

 top of the tower of the church of St. Augustine, had acquired mag- 

 netic properties. In 1558 John Baptista Porta, the reputed inventor 

 of the magic lantern, published a work on natural magic, the seventh 

 chapter of which is devoted to the magnet, and to the tricks which 

 may be played by means of it. Porta added a little to previous 

 knowledge. He speaks (I quote, however, from the subsequent 

 edition of 165 1) of the two poles of the loadstone, which he some- 

 times speaks of as the boreal and austral poles, and sometimes as 

 the arctic and antarctic poles of the stone. He gave a method of 

 finding the position of the poles on the stone. He was also aware 

 that a loadstone when divided into two parts becomes two complete 

 loadstones. He mentions that the magnetism produced in a piece 

 of iron by rubbing the end of it with the north pole of a loadstone is 

 diminished by subsequently rubbing the same end with the south 

 pole. He states that the only way of destroying the magnetism of a 

 magnet is by heating it with fire. He also combated a fal)le handed 



