Finding the surface of a sphere
Diameter 7, great circle 22: their product 154 gives the surface
A plain pen circle divided by a vertical diameter, labelled 22 above (the great circle) and 7 within (the diameter). The note asks for the surface of a spherical body and gives the rule: with diameter 7 and greatest circle of the ball 22, multiply 7 by 22 to get 154, which is the required surface of the sphere.
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Surface of a sphere from diameter and great circle
The single circle stands for the ball, its vertical line the diameter, marked 7, and the circle itself standing for the greatest circle, marked 22. The problem set beside it is to measure the outer surface of that sphere. It is one of a run of pocket rules turning a ball's linear measures into its area and volume.
Worked rule: diameter x great circle = surface
Leonardo takes the diameter as 7 and the greatest circle of the ball as 22. He multiplies '7 times 22 makes 154', and declares 154 to be the sought surface of the sphere. The numbers 22 and 7 double as the ratio he uses throughout these folios for the circle.