As usual, he combined experience and experiment; in fact he used the same word, esperienza, for both. While in Florence, he devised a pair of goggles for his dives in the Arno so he could study the water as it flowed past a weir. He threw oak apples or corks into a river and counted "beats of time" to study how long it took those in the center and those nearer the banks to move two hundred feet. He made floats that could hover at different depths to see how the currents changed from the surface to the bottom, and he crafted instruments that could measure a river's downhill course so he could determine the "rate of fall of a river per mile.
( Walter Isaacson )
[ Leonardo da Vinci ]
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