shall perceive that they may be applied the one to the other; for, having placed PA upon its equal QF, the side PC will fall upon its equal QD, and thus the two triangles will coincide; consequently they are equal, and the surface DQF=APC. For a similar reason the surface FQE = CPB, and the surface DQE = APB; we have accordingly

=

DQF+FQE-DQE APC + CPB-APB, or DEF = ABC; therefore the two symmetrical triangles ABC, DEF, are equal in surface.

498. Scholium. The poles P and Q may be situated within the triangles ABC, DEF; then it would be necessary to add the three triangles DQF, FQE, DQE, in order to obtain the triangle DEF, and also the three triangles APC, CPB, APB, in order to obtain the triangle ABC. In other respects the demonstration would always be the same and the conclusion the same.

Fig. 238.

in

THEOREM.

499. If two great circles AOB, COD (fig. 238), cut each other any manner in the surface of a hemisphere AOCBD), the sum of the opposite triangles AOC, BOD, will be equal to the lunary surface of which the angle is BOD.

Demonstration. By producing the arcs OB, OD, into the surface of the other hemisphere till they meet in N, OBN will be a semicircumference as well as AOB; taking from each OB, we shall have BN AO. For a similar reason DN CO, and BD=AC; consequently the two triangles AOC, BDN, have the three sides of the one equal respectively to the three sides of the other; moreover, their position is such that they are symmetrical; therefore they are equal in surface (496), and the sum of the triangles AOC, BOD, is equivalent to the lunary surface OBNDO, of which the angle is BOD.

500. Scholium. It is evident also that the two spherical pyramids, which have for their bases the triangles AOC, BOD, taken together, are equal to the spherical wedge of which the angle is BOD:

THEOREM.

501. The surface of a spherical triangle has for its measure the excess of the sum of the three angles over two right angles.

Demonstration. Let ABC (fig. 239) be the triangle proposed; Fig. 239. produce the sides till they meet the great circle DEFG drawn at pleasure without the triangle. By the preceding theorem the two triangles ADE, AGH, taken together, are equal to the lunary surface of which the angle is A, and which has for its measure 2A (495); thus we shall have ADE + AGH = 2A; for a similar reason BGF+ BID = 2B, CIH + CFE = 2C. But the sum of these six triangles exceeds the surface of a hemisphere by twice the triangle ABC; moreover the surface of a hemisphere is represented by 4; consequently the double of the triangle ABC is equal to 24 + 2B + 2C — 4, and consequently ABC=A+B+C-2; therefore every spherical triangle has for its measure the sum of its angles minus two right angles.

502. Corollary 1. The proposed triangle will contain as many triangles of three right angles, or eighths of the sphere (494), as there are right angles in the measure of this triangle. If the angles, for example, are each equal to of a right angle, then the three angles will be equal to four right angles, and the proposed triangle will be represented by 4-2 or 2; therefore it will be equal to two triangles of three right angles, or to a fourth of the surface of the sphere.

503. Corollary 11. The spherical triangle ABC is equivalent A+B+C to a lunary surface, the angle of which is 2

-

1; likewise the spherical pyramid, the base of which is ABC, is equal

to the spherical wedge, the angle of which is

A+B+C
2

1.

504. Scholium. At the same time that we compare the spherical triangle ABC with the triangle of three right angles, the spherical pyramid, which has for its base ABC, is compared with the pyramid which has a triangle of three right angles for its base, and we obtain the same proportion in each case. The solid angle at the vertex of a pyramid is compared in like manner with the solid angle at the vertex of the pyramid having a triangle of three right angles for its base. Indeed the comparison is established by the coincidence of the parts. Now, if the bases of pyramids coincide, it is evident that the pyramids themselves will coincide, as also the solid angles at the vertex. Whence we derive several consequences;

Fig. 240.

1. Two spherical triangular pyramids are to each other as their bases; and, since a polygonal pyramid may be divided into several triangular pyramids, it follows that any two spherical pyramids are to each other as the polygons which constitute their bases.

2. The solid angles at the vertex of these same pyramids are likewise proportional to the bases; therefore, in order to compare any two solid angles, the vertices are to be placed at the centres of two equal spheres, and these solid angles will be to each other as the spherical polygons intercepted between their planes or faces.

The angle at the vertex of the pyramid, whose base is a triangle of three right angles, is formed by three planes perpendicular to each other; this angle, which may be called a solid right angle, is very proper to be used as the unit of measure for other solid angles. This being supposed, the same number, which gives the area of a spherical polygon, will give the measure of the corresponding solid angle. If, for example, the area of a spherical polygon is, that is, if it is of a triangle of three right angles, the corresponding solid angle will also be of a solid right angle.

THEOREM.

505. The surface of a spherical polygon has for its measure the sum of its angles minus the product of two right angles by the number of sides in the polygon minus two.

Demonstration. From the same vertex A (fig. 240) let there be drawn to the other vertices the diagonals AC, AD; the polygon ABCDE will be divided into as many triangles minus two as it has sides. But the surface of each triangle has for its measure the sum of its angles minus two right angles, and it is evident that the sum of all the angles of the triangles is equal to the sum of the angles of the polygon; therefore the surface of the polygon is equal to the sum of its angles diminished by as many times two right angles as there are sides minus two.

506. Scholium. Let s be the sum of the angles of a spherical polygon, n the number of its sides; the right angle being supposed unity, the surface of the polygon will have for its measure s-2(n-2) or s-2n+4.

SECTION FOUTH.

Of the Three Round Bodies.

DEFINITIONS.

507. We call a cylinder the solid generated by the revolution of a rectangle ABCD (fig. 250), which may be conceived to Fig. 250. turn about the side AB considered as fixed.

During this revolution the sides AD, BC, remaining always perpendicular to AB, describe equal circular planes DHP, CGQ, which are called the bases of the cylinder, and the side CD describes the convex surface of the cylinder.

The fixed line AB is called the axis of the cylinder.

Every section KLM, made by a plane perpendicular to the axis, is a circle equal to each of the bases; for, while the rectangle ABCD turns about AB, the line IK, perpendicular to AB, describes a circular plane equal to the base, and this plane is simply the section made perpendicular to the axis at the point I. Every section PQGH, made by a plane passing through the axis, is a rectangle double of the generating rectangle ABCD.

508. We call a cone the solid generated by the revolution of a right-angled triangle SAB (fig. 251), which may be conceived to Fig. 251. turn about the fixed side SA.

In this revolution the side AB describes a circular plane BDCE called the base of the cone, and the hypothenuse SB describes the convex surface of the cone.

The point S is called the vertex of the cone, SA the axis or altitude, and SB the side.

Every section HKFI, made perpendicularly to the axis, is a circle; every section SDE, made through the axis, is an isosceles triangle double of the generating triangle SAB.

509. If from the cone SCDB we separate by a section parallel to the base the cone SFKH, the remaining solid CBHF is called a truncated cone or a frustum of a cone. It may be conceived to be generated by the revolution of the trapezoid ABHG, of which the angles A and G are right angles, about the side AG. The fixed line AG is called the axis or altitude of the frustum, the circles BDC, HKF, are the bases and BH the side of the frustum.

Fig. 252.

Fig. 253.

Fig. 254.

510. Two cylinders or two cones are similar, when their axes are to each other as the diameters of their bases.

511. If, in the circle ACD (fig. 252), considered as the base of a cylinder, a polygon ABCDE be inscribed, and upon the base ABCDE a right prism be erected equal in altitude to the cylinder, the prism is said to be inscribed in the cylinder, or the cylinder to be circumscribed about the prism.

It is manifest that the edges AF, BG, CH, &c., of the prism, being perpendicular to the plane of the base, are comprehended in the convex surface of the cylinder; therefore the prism and cylinder touch each other along these lines.

512. In like manner, if ABCD (fig. 253) be a polygon circumscribed about the base of a cylinder, and upon the base ABCD a right prism, equal in altitude to the cylinder, be constructed, the prism is said to be circumscribed about the cylinder, or the cylinder inscribed in the prism.

Let M, N, &c., be the points of contact of the sides AB, BC, &c., and through the points M, N, &c., let the lines MX, NY, &c., be drawn perpendicular to the plane of the base; it is evident that these perpendiculars will be in the surface of the cylinder and in that of the circumscribed prism at the same time; therefore they will be lines of contact.

N. B. The cylinder, the cone and the sphere are the three round bodies, which are treated of in the elements.

Preliminary Lemmas upon Surfaces.

513. 1. A plane surface OABCD (fig. 254) is less than any other surface PABCD terminated by the same perimeter ABCD.

Demonstration. This proposition is sufficiently evident to be ranked among the number of axioms; for we may consider the plane among surfaces what the straight line is among lines. The straight line is the shortest distance between two given points; in like manner the plane is the least surface among all those which have the same perimeter. Still, as it is proper to make the number of axioms as small as possible, I shall present a process of reasoning which will leave no doubt with regard to this proposition.

As a surface is extension in length and breadth, we cannot conceive one surface to be greater than another, except the di