o) 
50 
NATURE 
[JuLy 10, 1902 
or heat whether dry or suffused with moisture invites 
magneticks, even with the most solid bodies interposed, 
even planks of wood or pretty thick slabs of stone or 
sheets of metal. A loadstone appeals to magneticks only, 
towards electricks all things move.” 
He has no mercy on those who would make a perpetual- 
motion machine by means of the attraction of a load- 
stone. 
“ But they have been little practised in magnetick ex- 
periments who forge such things as that... . Oh that 
the gods would at length bring to a miserable end such | 
fictitious, crazy, deformed labours with which the minds 
of the studious are blinded.” 
Book iii. is on Direction, the property of the magnet 
to point north and south. At the outset Gilbert recognises 
that the compass needle deviates from the true North 
Pole by an amount which varies at different points on the 
the name given to the property of pointing north and 
south. 
Book iv. deals with ‘ Variation,” the angle be- 
tween the true and magnetic meridian at any point, 
and though we cannot agree with Gilbert that “the 
variation is caused by the inequality of the projecting 
parts of the earth,” or that “the variation in any one 
place is constant,” we can admire his skill and resource 
in utilising the scanty material at his disposal and in 
devising methods to measure the amount of the variation. 
In Book y. the action of a dipping needle is described 
and explained, while Book vi. treats of the “ Globe of 
the Earth the Great Magnet.” 
Any notice of this edition of the “De Magnete” 
would be incomplete without some reference to the notes 
contributed by the editor. 
During the work of revising and editing the English 
NW 
AW 
Fic. 1.—The Blacksmith making a Magnet. 
earth. ‘But it must be understood,” he says, “on the 
threshold of the argument (before we proceed further) 
that these pointings of the loadstone or of iron are not 
perpetually made toward the true poles of the world, do 
not always seek those fixed and definite points or remain 
on the line of the true meridian, but usually diverge some 
distance to the east or west.” 
The fundamental laws of the magnetisation of iron by 
contact with another magnet by induction either from a 
loadstone or in the earth’s field are clearly set out. 
Gilbert knew, too, how to demagnetise a magnet. 
“Putting the whole iron in the fire,” he writes, ‘ blow 
the fire with the bellows so that it may be all aglow and 
let it remain a little longer time red hot. When cooled 
(so, however, that while it is cooling it does not rest in 
one position) you will see that it has lost the 
verticity it had acquired from the stone.” Verticity is 
NO. 1706, VOL. 66] 
translation of “De Magnete,” many points, as Prof. 
Thompson writes, came up for discussion requiring 
critical consideration and the examination of the writings 
of contemporary or earlier authorities. The results of 
some portion of this labour have been collected in the 
form of notes. The text has with great judgment been 
printed just as Gilbert left it ; in fact, comparison shows 
that throughout the English and the original Latin 
versions run page for page. The notes cover some 
seventy pages, and are replete with curious and interest- 
ing information. Take, for example, that relating to the 
picture of the blacksmith striking the iron while it lies 
north and south, given on p. 139, which we have repro- 
duced. It appears that woodcuts containing human 
figures are rare in the art of the sixteenth century, and 
Prof. Thompson traces Gilbert’s picture to a book of fables 
by Cornelius van Kiel, published at Cologne in 1594, 
