[This] is an enlarged illustration and detailed description of water in percussion: that is, water is itself the obstacle to water, and in this case the contrast is between the resulting currents on the surface, under the surface, and surging upward carrying bubbles of entrapped air. The relationship between air and water, both in combination and as analogous media, was also a subject that greatly preoccupied Leonardo and played a critical role in the development of his thought that concerns me here.
As frequently happens with Leonardo, his observations are truly amazing. I once showed a group of the drawings to a hydraulic engineer here in Princeton, who having studied them for several weeks, was also astonished by them, reporting that Leonardo evidently saw with the naked eye, as it were, some actions of water that have been confirmed only in modern times with sophisticated methods of observation, measurement, and control. Such observations are popularly attributed to a superior physiological apparatus with which Leonardo was endowed, but I doubt it. I suspect that his studies are just as much the product of cerebration as of observation, deductions or inductions from what can be seen to take place to what must be assumed to take place.
Irving Lavin, Institute for Advanced Study, Princeton. April 21, 1993.