ON MOVEMENTS;--TO KNOW HOW MUCH A SHIP ADVANCES IN AN HOUR.

The ancients used various devices to ascertain the distance gone by
a ship each hour, among which Vitruvius [Footnote 6: See VITRUVIUS,
_De Architectura lib. X._ C. 14 (p. 264 in the edition of Rose and
Muller- Strubing). The German edition published at Bale in 1543 has,
on fol. 596, an illustration of the contrivance, as described by
Vitruvius.] gives one in his work on Architecture which is just as
fallacious as all the others; and this is a mill wheel which touches
the waves of the sea at one end and in each complete revolution
describes a straight line which represents the circumference of the
wheel extended to a straightness. But this invention is of no worth
excepting on the smooth and motionless surface of lakes. But if the
water moves together with the ship at an equal rate, then the wheel
remains motionless; and if the motion of the water is more or less
rapid than that of the ship, then neither has the wheel the same
motion as the ship so that this invention is of but little use.
There is another method tried by experiment with a known distance
between one island and another; and this is done by a board or under
the pressure of wind which strikes on it with more or less
swiftness. This is in Battista Alberti [Footnote 25: LEON BATTISTA
ALBERTI, _De Architectura lib. V._, c. 12 treats '_de le navi e
parti loro_', but there is no reference to the machine, mentioned by
Leonardo. Alberti says here: _Noi abbiamo trattato lungamente in
altro luogo de' modi de le navi, ma in questo luogo ne abbiamo detto
quel tanto che si bisogna_. To this the following note is added in
the most recent Italian edition: _Questo libro e tuttora inedito e
porta il titolo, secondo Gesnero di_ '_Liber navis_'.].

Battista Alberti's method which is made by experiment on a known
distance between one island and another. But such an invention does
not succeed excepting on a ship like the one on which the experiment
was made, and it must be of the same burden and have the same sails,
and the sails in the same places, and the size of the waves must be
the same. But my method will serve for any ship, whether with oars
or sails; and whether it be small or large, broad or long, or high
or low, it always serves [Footnote 52: Leonardo does not reveal the
method invented by him.].

Methods of staying and moving in water