The ship’s logs of Vitruvius, of Alberti and of Leonardo

1113.

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

1114.

How an army ought to cross rivers by swimming with air-bags ...  How fishes swim [Footnote 2:  Compare No. 821.]; of the way in which they jump out of the water, as may be seen with dolphins; and it seems a wonderful thing to make a leap from a thing which does not resist but slips away.  Of the swimming of animals of a long form, such as eels and the like.  Of the mode of swimming against currents