5. The plaine of the base and heighth is the solidity of a right prisma.
6. A prisma is the triple of a pyramis of equall base and heighth. è 7 p. xij.
As in the example a prisma pentaedrum is cut into three equall pyramides. For the first consisting of the plaines aei, aeo, aoi, eio; is equall to the second consisting of the plaines aoi, aou, aiu, iou, by the [10 e vij]. Because it is equall to it both in common base and heighth. Therefore the first and second are equall. And the same second is equall to it selfe, seeing the base is iou, and the toppe a. Then also it is equall to the third consisting of the plaines aiu, aiy, uiy, auy. Therefore three are equall.
If the base be triangular, the Prisma may be resolved into prisma's of triangular bases, and the theoreme shall be concluded as afore.
Therefore
7. The plaine made of the base and the third part of the heighth is the solidity of a pyramis of equall base and heighth.
The heighth of a pyramis shall be found, if you shall take the square of the ray of the base out of the quadrate of the side: for the side of the remainder, by the [9 e xij], shall be the altitude or heighth, as in the example following.
