er written by Peter Adsiger, in the
year 1269. This letter is preserved among the manuscripts of the university
of Leyden; extracts from it are given by Cavallo, in the second edition of
his Treatise on Magnetism. From these extracts it is evident that he was
acquainted with the attraction, repulsion, and polarity of the magnet, the
art of communicating those properties to iron, the variation of the
magnetic needle; and there are even some indications that he was acquainted
with the construction of the azimuth compass.

Next in importance and utility to the mariners' compass, in preparing the
way for the great discoveries by which the fifteenth century is
distinguished, maps and charts may be placed. For though, in general, they
were constructed on very imperfect and erroneous notions of the form of the
world, and the relative size and situation of different countries, yet
occasionally there appeared maps which corrected some long established
error, or supplied some new information; and even the errors of the maps,
in some cases, not improbably held out inducements or hopes, which would
not otherwise have been entertained and realized, as we have already
remarked, after D'Anville, that the greatest of Ptolemy's errors proved
eventually the efficient cause which led to the greatest discovery of the
moderns.

Malte Brun divides the maps of the middle ages into two classes: those in
which the notions of Ptolemy and other ancient geographers are implicitly
copied, and those in which new countries are inserted, which had been
either discovered, or were supposed to exist.

In most of the maps of the first description, Europe, Asia, and Africa are
laid down as forming one immense island, and Africa is not carried so far
as the equator. One of the most celebrated of these maps was drawn up by
Marin Sanuto, and inserted in his memorial presented to the pope and the
principal sovereigns of Europe, for the purpose of persuading and shewing
them, that if they would oblige their merchants to trade only through the
dominions of the Caliphs of Bagdat, they would be better supplied and at a
cheaper rate, and would have no longer to fear the Soldans of Egypt. This
memorial with its maps was inserted in the Gesta Dei per Francos, as we are
assured by the editor, from one of the original copies presented by Sanuto
to some one of the princes. Hence, as Dr. Vincent remarks, it probably
contains the oldest map of the world at this day extan