On the definition of surface
Surface as the common boundary between air and the bodies it clothes
A small leaf given over almost entirely to mirror-written prose, with a single small pen sketch in the upper right corner that the bundle does not transcribe. Across several restarted attempts Leonardo works toward a definition of 'surface,' settling on it as the contingency of the extremities of bodies — the common boundary that finishes and terminates the air and the bodies clothed by it, partaking of neither and dividing one from the other. The untranscribed corner sketch is present on the sheet but not included in the transcription.
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Surface as the common boundary of bodies
A surface is the contingency of the extremities of bodies — made where the extremities of the body of air meet the extremities of the bodies clothed by that air. It finishes and terminates the surrounded bodies with the air, and the air with the bodies, partaking of neither the surrounding nor the surrounded body. Rather it is the proper common boundary of each, dividing one body from another, as air or water from the body enclosed within them.
Small untranscribed corner sketch
A small pen drawing of an angular, folded form sits in the upper right of the sheet. It is not included in the transcription, so its precise subject is left undescribed here beyond noting its presence.