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angle DEF; therefore as the triangle ABC to the triangle

R DEF, so is the altitude of the

K solid EHPO to the altitude of the solid BGML: but the altitude of the solid EHPO is the same with the altitude of the pyramid DEFH; and the altitude of the solid BGML is the same with the altitude of the pyramid ABCG; therefore, as the base ABC to the base DEF, so is the altitude of the pyramid DEFH to the altitude of the pyramid ABCG: wherefore, the bases and altitudes of the pyramids A BCG, DEFH, are reciprocally proportional.

Again, let the bases and altitudes of the pyramids ABCG, DEFH, be reciprocally proportional, viz. the base ABC to the base DEF, as the altitude of the pyramid DEFH to the altitude of the pyramid ABCG: the pyramid ABCG shall be equal to the pyramid DEFH.

The same construction being made, because as the base ABC to the base DEF, so is the altitude of the pyramid DEFH to the altitude of the pyramid ABCG; and as the base ABC to the base DEF, so is the parallelogram BM to the parallelogram EP: therefore the parallelogram BM is to EP, as the altitude of the pyramid DEFH to the altitude of the pyramid ABCG: but the altitude of the pyramid DEFH is the same with the altitude of the solid parallelopiped EHPO; and the altitude of the pyramid ABCG is the same with the-altitude of the solid parallelopiped BGML: therefore as the base BM to the base EP, so is the altitude of the solid parallelopiped EHPO to the altitude of the solid parallelopiped BGML: but solid parallelopipeds having their bases and altitudes reciprocally proportional, are equal * to one another; therefore the solid . 34. 11. parallelopiped BGML is equal to the solid parallelopiped EHPO: and the pyramid ABCG is the sixth part of the solid BGML, and the pyramid DEFH is the sixth part of the solid EHPO; therefore the pyramid ABCG is equal to the pyramid DEFH. There- ti2 Ax. 5. fore the bases, &c. Q. E. D.

PROP. X. THEOR,

Every cone is the 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 shall be the third part of the cylinder; that is, the cylinder shall be 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 inscribe 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 shall be greater than half of the cylinder: for let a square be described about the circle, and let a prism be erected upon the square, of the same altitude with the cylinder : then the inscribed square is balf 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 the half of the prism upon the square described about the circle; because they are to one another* as their bases ; 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 the half of the segment of the circle in which it stands, as was shewn in Prop. 2, of this Book. Erect prisms upon each of these triangles, of

Η the same altitude with the cylinder; each of these prisms shall be 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

* 32. 11.

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As was shewn in Prop. 2. of this Book.

upon the same AB, BC, CD, DA, and solid parallelo, pipeds be erected upon the parallelograms; the prisms upon the triangles AEB, BFC, CGD, DHA, are the halves of the solid parallelopipeds *; and the segments * 2 Cor. 7. of the cylinder which are upon the segments of the 12. 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: therefore, 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 * than the excess

* Lemma. 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* 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: therefore the cylinder is not greater than the triple of the cone. Nor can the cylinder be less than the triple of the

Let it be less, if possible; therefore, inversely, the cone is greater than the third part of the cylinder. In the circle ABCD inscribe a square : this square is greater than the half of the circle: and

upon

the

square ÅBCD 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 upon these squares there be erected solid parallelopipeds of the same altitude with the cone, which are also prisnis, the prism upon the square ABCD is the half of that which is upon the square

1 Cor. 7. 12,

cone.

E

described about the circle; for they are * 32. 11. to one another as their bases *; as are

also the third parts of them: therefore
the pyramid, the base of which is the
square ABCD, is half of the pyramid
upon the square described about the
circle: but this last pyramid is greater
than the cone which it contains; there-
fore 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 yertex with the cone: therefore each of those 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 † Lemma. , together are lesst 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 : there-
fore the rest of the cone, that is, the pyramid of which
the base is the polygon AEBFCGDH, and of which
the vertex is the same with that of the
cone, is greater than the third part of
the cylinder: but this pyramid is the
third part of the prism upon the same
base AEBFCGDH, and 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 also
less, for it is contained within the cylinder; which is
impossible: 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.

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PROP. XI. THEOR.

Cones and cylinders of the same altitude, are to one an- See N.

other 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 shall the cone AL be to the cone EN.

If it be not so, the circle ABCD must 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 inscribe 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 shall be 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 5, the pyramid inscribed in the cone is half of the pyramid circumscribed about it, because they are to one another as their bases *: 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

N half of the cone.

Divide

V

R the circumferences EF, FG, GH, HE, each into two

G equal parts in the points O,

'T P, R, S, and join EO,

B 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

* 6. 12.

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А

CE

# Vertex is put in 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.

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