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But CP being drawn perpendicular to AB, the triangles ABO
AO : CP :: AB : CD;
but CP x AB is double the area of the triangle ABC; hence we have
hence the solid described by the triangle ABC is also measured by ABC x IK, or which is the same thing, by ABC × circ. IK, circ. IK being equal to 2x IK. Hence the solid described by the C revolution of the triangle ABC,has for its measure the area of this triangle multiplied by two thirds of the circumference traced by I, the middle point of the base.
Cor. If the side AC=CB, the line CI will be perpendicular to AB, the area ABC will be equal to AB×CI, and the solidity × ABC X IK will become x ABX IK CI. But the triangles ABO, CIK, are similar, and give the proportion AB BO or MN CI: IK; hence AB×IK=MN× CI; hence the solid described by the isosceles triangle ABC will have for its measure ×CI2× MN: that is, equal to two thirds of π τητο the square of the perpendicular let fall on the base, into the distance between the two perpendiculars let fall on the axis.
Scholium. The general solution appears to include the supposition that AB produced will meet the axis; but the results would be equally true, though AB were parallel to the axis. Thus, the cylinder described by AMNB P is equal to 7.AM2.MN; the cone described by ACM is equal to 7.AM2.CM, and the cone described by BCN to
AM2 CN. Add the first two solids and take away the third; we shall have the solid described by ABC equal to π.АM2. (MN+CM-CN): and since CN-CM=MN, this expression is reducible to 7.AM2. MN, or 37.CP2.MN: which agrees with the conclusion found above.
PROPOSITION XIII. LEMMA.
If a regular semi-polygon be revolved about a line passing through the centre and the vertices of two opposite angles, the solid described will be equivalent to a cone, having for its base the inscribed circle, and for its altitude twice the axis about which the semi-polygon is revolved.
Let the semi-polygon FABG be revolved about FG: then, if OI be the radius of the inscribed circle, the solid described will be measured by area OI × 2FG.
For, since the polygon is regular, the triangles OFA, OAB, OBC, &c. are equal C and isosceles, and all the perpendiculars let fall from O on the bases FA, AB, &c. will be equal to OI, the radius of the inscribed circle.
PROPOSITION XIV. THEOREM.
Now, the solid described by OAB is measured by O12+ MN (Prop. XII. Cor.) ; the solid described by the triangle OFA has for its measure OI2 FM, the solid described by the triangle OBC, has for its measure OI2× NO, and since the same may be shown for the solid described by each of the other triangles, it follows that the entire solid described by the semi-polygon is measured by 7012.(FM+MN+NO+OQ+QG), or OI2 × FG; which is also equal to OI2 × 2FG. But .OI2 is the area of the inscribed circle (Book V. Prop. XII. Cor. 2.): hence the solidity is equivalent to a cone whose base is area OI, and altitude 2FG.
The solidity of a sphere is equal to its surface multiplied by a third of its radius.
Inscribe in the semicircle ABCDE a regular semi-polygon, having any number of sides, and let OI be the radius of the circle inscribed in the polygon.
If the semicircle and semi-polygon be revolved about EA, the semicircle will C describe a sphere, and the semi-polygon a solid which has for its measure 37ŎI2× EA (Prop. XIII.); and this will be true. whatever be the number of sides of the polygon. But if the number of sides of the polygon be indefinitely increased, the semi-polygon will become the semicircle, OI will become equal to OA, and the solid described by the semi-polygon will become the sphere: hence the solidity of the sphere is equal to πOA2× EA, or by substituting 20A for EA, it becomes 1.OA2× OA, which is also equal to 470A2× OA. But 47.OA2 is equal to the surface of the sphere (Prop. X. Cor.): hence the solidity of a sphere is equal to its surface multiplied by a third of its radius.
Scholium 1. The solidity of every spherical sector is equal to the zone which forms its base, multiplied by a third of the radius. For, the solid described by any portion of the regular polygon, as the isosceles triangle OAB, is measured by 2¬OI2 × AF (Prop. XII. Cor.); and when the polygon becomes the circle, the portion OAB becomes the sector AOB, OI becomes equal to OA, and the solid described becomes a spherical sector. But its measure then becomes equal to 27.AO2× AF, which is equal to 27.AO × AF × AO. But 2.AO is the circumference of a great circle of the sphere (Book V. Prop. XII. Cor. 2.), which being multiplied by AF gives the surface of the zone which forms the base of the sector (Prop. X. Sch. 1.): and the proof is equally applicable to the spherical sector described by the circular sector BOC: hence, the solidity of the spherical sector is equal to the zone which forms its base, multiplied by a third of the radius.
Scholium 2. Since the surface of a sphere whose radius is R. is expressed by 47R2 (Prop. X. Cor.), it follows that the surfaces of spheres are to each other as the squares of their radii; and since their solidities are as their surfaces multiplied by their radii, it follows that the solidities of spheres are to each other as the cubes of their radii, or as the cubes of their diameters.
Scholium 3. Let R be the radius of a sphere; its surface will be expressed by 47R2, and its solidity by 4лR2 × R, or R3. If the diameter is called D, we shall have R=D, and R3=1D3: hence the solidity of the sphere may likewise be expressed by
PROPOSITION XV. THEOREM.
The surface of a sphere is to the whole surface of the circumscribed cylinder, including its bases, as 2 is to 3: and the solidities of these two bodies are to each other in the same ratio.
Let MPNQ be a great circle of the sphere; ABCD the circumscribed square if the semicircle PMQ and the half square PADQ are at the same time made to revolve about the diameter PQ, the semicircle will gene- M rate the sphere, while the half square will generate the cylinder circumscribed about that sphere.
The altitude AD of the cylinder is equal to the diameter PQ; the base of
the cylinder is equal to the great circle, since its diameter AB is equal to MN; hence, the convex surface of the cylinder is equal to the circumference of the great circle multiplied by its diameter (Prop. 1.). This measure is the same as that of the surface of the sphere (Prop. X.): hence the surface of the sphere is equal to the convex surface of the circumscribed cylinder.
But the surface of the sphere is equal to four great circles; hence the convex surface of the cylinder is also equal to four great circles: and adding the two bases, each equal to a great circle, the total surface of the circumscribed cylinder will be equal to six great circles; hence the surface of the sphere is to the total surface of the circumscribed cylinder as 4 is to 6, or as 2 is to 3; which was the first branch of the Proposition.
In the next place, since the base of the circumscribed cylinder is equal to a great circle, and its altitude to the diameter, the solidity of the cylinder will be equal to a great circle multiplied by its diameter (Prop. II.). But the solidity of the sphere is equal to four great circles multiplied by a third of the radius (Prop. XIV.); in other terms, to one great circle multiplied by of the radius, or by of the diameter; hence the sphere is to the circumscribed cylinder as 2 to 3, and conse quently the solidities of these two bodies are as their surfacer
Scholium. Conceive a polyedron, all of whose faces touch the sphere; this polyedron may be considered as formed of pyramids, each having for its vertex the centre of the sphere, and for its base one of the polyedron's faces. Now it is evident that all these pyramids will have the radius of the sphere for their common altitude: so that each pyramid will be equal to one face of the polyedron multiplied by a third of the radius: hence the whole polyedron will be equal to its surface multiplied by a third of the radius of the inscribed sphere.
It is therefore manifest, that the solidities of polyedrons circumscribed about the sphere are to each other as the surfaces of those polyedrons. Thus the property, which we have shown to be true with regard to the circumscribed cylinder, is also true with regard to an infinite number of other bodies.
We might likewise have observed that the surfaces of polygons, circumscribed about the circle, are to each other as their perimeters.
PROPOSITION XVI. PROBLEM.
If a circular segment be supposed to make a revolution about a diameter exterior to it, required the value of the solid which it describes.
Let the segment BMD revolve about AC. On the axis, let fall the perpendiculars BE, DF; from the centre C, draw CI perpendicular to the chord BD; also draw the radii CB, CD.
The solid described by the sector BCD is measured by 7 CB2.EF (Prop. XIV. Sch. 1). But the solid described by the isosceles triangle DCB has for its measure .CI.EF (Prop. XII. Cor.); hence the solid described by the segment BMD=37.EF.(CB2-CI). Now, in the rightangled triangle CBI, we have CB2-CI-BIBD2; hence the solid described by the segment BMD will have for its measure 7.EF.BD2, or 17.BD2.EF: that is one surth of π ιnto the square of the chord, into the distance between the two perpendiculars let fall from the extremities of the arc on the axis.
Scholium. The solid described by the segment BMD is to the sphere which has BD for its diameter, as .BD2.EF is to 7.BD3, or as EF to BD.