The centres of the sphere of water are two, one universal and common
to all water, the other particular. The universal one is that which
is common to all waters not in motion, which exist in great
quantities. As canals, ditches, ponds, fountains, wells, dead
rivers, lakes, stagnant pools and seas, which, although they are at
various levels, have each in itself the limits of their superficies
equally distant from the centre of the earth, such as lakes placed
at the tops of high mountains; as the lake near Pietra Pana and the
lake of the Sybil near Norcia; and all the lakes that give rise to
great rivers, as the Ticino from Lago Maggiore, the Adda from the
lake of Como, the Mincio from the lake of Garda, the Rhine from the
lakes of Constance and of Chur, and from the lake of Lucerne, like
the Tigris which passes through Asia Minor carrying with it the
waters of three lakes, one above the other at different heights of
which the highest is Munace, the middle one Pallas, and the lowest
Triton; the Nile again flows from three very high lakes in Ethiopia.

[Footnote 5: _Pietra Pana_, a mountain near Florence. If for Norcia,
we may read Norchia, the remains of the Etruscan city near Viterbo,
there can be no doubt that by '_Lago della Sibilla_'--a name not
known elsewhere, so far as I can learn--Leonardo meant _Lago di
Vico_ (Lacus Ciminus, Aen. 7).]