Vol. I: VII. On the Proportions and on the Movements of t...
The Da Vinci Notebooks
When two men are at the opposite ends of a plank that is balanced, and if they are of equal weight, and if one of them wants to make a leap into the air, then his leap will be made down from his end of the plank and the man will never go up again but must remain in his place till the man at the other end dashes up the board.
199:191 : See Pl. XXIV, No. 3.