Why the Arrow Leaves at the End of the Cord's Motion
Impetus of the crossbow cord; a weight balanced over two pulleys (4 & 4, then 4 & 8)
Under the heading On the motion of the arrow, Leonardo observes that although the crossbow's force is greatest at the start and nil at the last, the cord's motion actually grows faster toward the end because of the impetus it has gathered; he concludes that the arrow parts at the end of the cord's travel. Above the text a spherical weight is slung by cords that run over a pulley on either side to two more suspended weights. When both side weights read 4 the central weight hangs straight down; when they read 4 and 8 the suspending cord skews toward the heavier side.
On this page
The arrow parts at the end of the cord's motion
Although the crossbow's force is great at the beginning and nothing at the last, the motion of the cord grows faster toward the end than at the beginning because of the impetus acquired. It follows that the arrow leaves at the end of the cord's motion.
A weight balanced over two pulleys
A central sphere is held by cords passing over a pulley on either side to two more weights. With equal side weights (4 and 4) the central weight hangs vertically; with unequal ones (4 and 8) the suspending cord leans toward the heavier weight, so the central weight no longer hangs straight below its attachment.