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
Caesar, 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 1651) 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 fable handed