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is the sixth part of the solid BGML, and the pyramid DEFH is the sixth part of the solid EPHO. Therefore the pyramid ABCD is equal to the pyramid DEFH. Therefore the bases, &c. Q. E. D.

PROP. X. THEOR.

EvKRY cone is a third part of a cylinder which has the same base, and is of an equal altitude with it.

Let a cone have the same base with a cylinder, viz. the circle ABCD, and the same altitude. The cone is the third part of the cylinder; that is, the cylinder is triple of the cone.

If the cylinder be not triple of the cone, it must either be greater than the triple, or less than it. First, let it be greater than the triple: and describe the square ABCD in the circle; this square is greater than the half of the circle ABCD:* Upon the square ABCD erect a prism of the same altitude with the cylinder; this prism is greater than half of the cylinder: because if a square be described about the

circle, and a prism erected upon the square, of the same altitude

with the cylinder, the inscribed square is half of that circumscribed; and upon these square bases are erected solid parallelopipeds, viz. the prisms of the same altitude; therefore the prism upon the square ABCD is half of the prism upon the square described about the circle: because they are to one another as their bases (32. 11.); and the cylinder is less than the prism upon the square described about the circle ABCD: therefore the prism upon the square ABCD of the same altitude with the cylinder, is greater than half of the cylinder. Bisect the circumferences AB, BC, CD, DA in the points E, F, G, H, and join AE, EB, BF, FC, CG, GD, DH, HA: then each of the triangles AEB, BFC, CGD, DHA is greater than half of the segment of the circle in which it stands, as was shöwn in prop. 2. of this book. Erect prisms upon each of these triangles, of the same altitude with the cylinder; each of these prisms is greater than half of the segment of the cylinder in which it is ; because if through the points E, F, G, H, parallels be drawn to AB, BC, CD, DA, and parallelograms be completed upon the same AB, BC, CD, DA, and solid parallelopipeds be erected upon the parallelograms; the prisms upon the triangles AEB, BFC, CGD, DHA are the halves of the solid parallelopipeds (2. Cor. 7. 12.). And the segments of the cylinder which are upon the segments of the circle cut off by AB, BC, CD, DA, are less than the solid parallelopipeds which contain them. Therefore the prisms upon the triangles AEB, BFC, CGD, DHA, are greater than half of the segments of the cylinder in which they are; there

* As was shown in prop. 2, of this book.

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fore if each of the circumferences be divided into two equal parts, and straight lines be drawn from the points of division to the extremities of the circumferences, and upon the triangles thus made, prisms be erected of the same altitude with the cylinder, and so on, there must at length remain some segments of the cylinder which together are less (Lem.) than the excess of the cylinder above the triple of the cone. , Let them be those upon the segments of the circle AE, EB, BF, FC, CG, GD, DH, HA. Therefore the rest of the cylinder, that is, the prism of which the base is the polygon AEBFCGDH, and of which the altitude is the same with that of the cylinder, is greater than the triple of the cone: but this prism is triple (1. Cor. 7. 12.) of the pyramid upon the same base, of which the vertex is the same with the vertex of the cone; therefore the pyramid upon the base AEBFCGDH, having the same vertex with the cone, is greater than the cone, of which the base is the circle ABCD: but it is also less, for the pyramid is contained within the cone; which is impossible. Nor can the cylinder be less than the triple of the cone. Let it be less, if possible: therefore, inversely, the cone is greater than the third part of the cylinder. In the circle ABCD describe a square; this square is greater than the half of the circle: and upon the square ABCD erect a pyramid having the same vertex with the cone: this pyramid is greater than the half of the cone; because, as was before demonstrated, if a square be described about the circle, the square ABCD is the half of it; and if, H upon these squares there be erected solid - parallelopipeds of the same altitudes with A. ID the cone, which are also prisms, the prism upon the square ABCD shall be the half of that which is upon the square described E o G about the circle; for they are to one another as their bases (32. 11.); as are also the third parts of them; therefore the pyramid, \ . the base of which is the square ABCD, is B\s]_2^C half of the pyramid upon the square de-, F scribed about the circle: but this last pyramid is greater than the cone which it contains; therefore the pyramid upon the square ABCD, having the same vertex with the cone, is greater than the half of the cone. Bisect the circumferences AB, BC, CD, DA in the points E, F, G, H, and join AE, EB, BF, FC, CG, GD, DH, HA: therefore each of the triangles AEB, BFC, CGD, DHA is greater than half of the segment of the circle in which it is: upon each of these triangles erect pyramids having the same vertex with the cone. Therefore each of these pyramids is greater than the half of the segment of the cone in which it is, as before was demonstrated of the prisms and segments of the cylinder: and thus dividing each of the circumferences into two equal parts, and joining the points of division and their extremities by straight lines, and upon the triangles erecting pyramids having their vertices the same with that of the cone, and so on, there must at length remain some segments of the cone, which together shall be less than the excess of the cone above the third part of the cylinder. Let these be the segments upon AE, EB, BF, FC, CG, GD, DH, HA. Therefore the rest of the cone, that is, the pyramid, of which the base is the H polygon AEBFCGDH, and of which the A vertex is the same with that of the cone, T) is greater than the third part of the cylinder. But this pyramid is the third part of the prism upon the same base AEBFCGDH, Eand of the same altitude with the cylinder. Therefore this prism is greater than the cylinder of which the base is the circle | ABCD. But it is o less; for it is con- B C tained within the cylinder; which is im- F possible. Therefore the cylinder is not less than the triple of the cone. And it has been demonstrated that neither is it greater than the triple. Therefore the cylinder is triple of the cone, or the cone is the third part of the cylinder. Wherefore every cone, &c. Q. E. D.

PROP. XI. THEOR.

CoNEs and cylinders of the same altitude, are to one another as their bases.”

Let the cones and cylinders, of which the bases are the circles ABCD, EFGH, and the axes KL, MN, and AC, EG the diameters of their bases, be of the same altitude. As the circle ABCD to the circle EFGH, so is the cone AL to the cone EN.

If it be not so, let the circle ABCD be to the circle EFGH, as the cone AL to some solid either less than the cone EN, or greater than it. First, let it be to a solid less than EN, viz. to the solid X; and let Z be the solid which is equal to the excess of the cone EN above the solid X; therefore the cone EN is equal to the solids X, Z together. In the circle EFGH describe the square EFGH, therefore this square is greater than the half of the circle: upon the square EFGH erect a pyramid of the same altitude with the cone; this pyramid is greater than half of the cone. For, if a square be described about the circle, and a pyramid be erected upon it, having the same vertex with the cone,f the pyramid inscribed in the cone is half of the pyramid circumscribed about it, because they are to one another as their bases (6.12.): but the cone is less than the circumscribed pyramid; therefore the pyramid of which the base is the square EFGH, and its vertex the same with that of the cone, is greater than half of the cone: divide the circumferences EF, FG, GH, HE, each into two

* See Note.

+ Vertex is put in the place of altitude, which is in the Greek, because the pyramid, in what follows, is supposed to be circumscribed about the cone, and so must have the same vertex. And the same change is made in some places following.

equal parts in the points O, P, R, S, and join EO, OF, FP, PG, GR, RH, HS, SE: therefore each of the triangles EOF, FPG, GRH, HSE is greater than half of the segment of the circle in which it is: upon

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each of these triangles erect a pyramid having the same vertex with the cone; each of these pyramids is greater than the half of the segment of the cone in which it is: and thus dividing each of these circumferences into two equal parts, and from the points of division drawing straight lines to the extremoties of the circumferences, and upon each of the triangles thus made erecting pyramids, having the same vertex with the cone, and so on, there must at length remain some segments of the cone which are together less (Lem. 1.) than the solid Z: let these be the segments upon EO, OF, FP, PG, GR, RH, HS, SE: therefore the remainder of the cone, viz. the pyramid of which the base is the polygon EOFPGRHS, and its vertex the same with that of the cone, is greater than the solid X: in the circle ABCD describe the polygon ATBYCVDQ similar to the polygon EOFPGRHS, and upon it erect a pyramid having the same vertex AL: and because as the square of AC is to the square of EG, so (l. 12.) is the polygon ATBYCVDQ to the polygon EOFPGRHS; and as the square of AC to the square of EG, so is (2.12.) the circle ABCD to the circle EFGH; therefore the circle ABCD (11. 5.) is to the circle EFGH, as the polygon ATBYCWDQ to the polygon EOFPGRHS: but as the circle ABCD to the circle EFGH, so is the cone AL to the solid X, and as the polygon ATBYCWDQ to the polygon EOFPGRHS, so is (6.12.) the pyramid of which the base is the first of these polygons, and vertex L, to the pyramid of which

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the base is the other polygon, and its vertex. N: therefore, as the cone AL to the solid X, so is the pyramid of which the base is the polygon ATBYCVDQ, and vertex L, to the pyramid the base of which is the polygon EOFPGRHS, and vertex N. : but the cone AL is greater than the pyramid contained in it; therefore the solid X is

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greater (14. 5.) than the pyramid in the come EN ; but it is less, as was shown, which is absurd : therefore the circle ABCD is not to the circle EFGH, as the cone AdL to any solid which is less than the cone EN. In the same manner it may be demonstrated that the circle EFGH is not to the circle ABCD, as the cone EN to any solid less than the cone AL. Nor can the circle ABCD be to the circle EFGH, as the cone AL to any solid greater than the cone EN: for, if it be possible, let it be so to the solid I, which is greater than the cone EN: therefore, by inversion, as the circle EFGH to the circle ABCD, so is the solid I to the cone AL : but as the solid I to the cone AL, so is the cone EN to some solid which must be less (14. 5.) than the cone AL, because the solid I is greater than the come EN : therefore as the circle EFGH is to the circle ABCD, so is the cone EN to a solid less than the cone AL, which was shown to be impossible; therefore the circle ABCD is not to the circle EFGH, as the cone AL is to any solid greater than the cone EN: and it has been demonstrated that neither is the circle ABCD to the circle EFGH, as the cone AL to any solid less than the cone EN: therefore the circle ABCD is to the circle EFGH, as the cone AL to the cone EN: but as the cone is to the cone, so (15. 5.) is the cylinder to the cylinder, because the cylinders are triple (10.

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