difficulties. They prove elaborately, by a reductio ad absurdum, that the volumes cannot be unequal. This proof must be read in the Elements. We must, however, state that we have in the above not proved Euclid’s Prop. 5, but only a special case of it. Euclid does not suppose that the bases of the two pyramids to be compared are equal, and hence he proves that the volumes are as the bases. The reasoning of the proof becomes clearer in the special case, from which the general one may be easily deduced.
§ 86. Prop. 6 extends the result to pyramids with polygonal bases. From these results follow again the rules at present given for the mensuration of solids, viz. a pyramid is the third part of a triangular prism having the same base and the same altitude. But a triangular prism is equal in volume to a parallelepiped which has the same base and altitude. Hence if B is the base and h the altitude, we have
| Volume of prism | = Bh, |
| Volume of pyramid | = 13Bh, |
statements which have to be taken in the sense that B means the number of square units in the base, h the number of units of length in the altitude, or that B and h denote the numerical values of base and altitude.
§ 87. A method similar to that used in proving Prop. 5 leads to the following results relating to solids bounded by simple curved surfaces:—
Prop. 10. Every cone is the third part of a cylinder which has the same base, and is of an equal altitude with it.
Prop. 11. Cones or cylinders of the same altitude are to one another as their bases.
Prop. 12. Similar cones or cylinders have to one another the triplicate ratio of that which the diameters of their bases have.
Prop. 13. If a cylinder be cut by a plane parallel to its opposite planes or bases, it divides the cylinder into two cylinders, one of which is to the other as the axis of the first to the axis of the other; which may also be stated thus:—
Cylinders on the same base are proportional to their altitudes.
Prop. 14. Cones or cylinders upon equal bases are to one another as their altitudes.
Prop. 15. The bases and altitudes of equal cones or cylinders are reciprocally proportional, and if the bases and altitudes be reciprocally proportional, the cones or cylinders are equal to one another.
These theorems again lead to formulae in mensuration, if we compare a cylinder with a prism having its base and altitude equal to the base and altitude of the cylinder. This may be done by the method of exhaustion. We get, then, the result that their bases are equal, and have, if B denotes the numerical value of the base, and h that of the altitude,
| Volume of cylinder | = Bh, |
| Volume of cone | = 13Bh. |
§ 88. The remaining propositions relate to circles and spheres. Of the sphere only one property is proved, viz.:—
Prop. 18. Spheres have to one another the triplicate ratio of that which their diameters have. The mensuration of the sphere, like that of the circle, the cylinder and the cone, had not been settled in the time of Euclid. It was done by Archimedes.
Book XIII.
§ 89. The 13th and last book of Euclid’s Elements is devoted to the regular solids (see Polyhedron). It is shown that there are five of them, viz.:—
1. The regular tetrahedron, with 4 triangular faces and 4 vertices;
2. The cube, with 8 vertices and 6 square faces;
3. The octahedron, with 6 vertices and 8 triangular faces;
4. The dodecahedron, with 12 pentagonal faces, 3 at each of the 20 vertices;
5. The icosahedron, with 20 triangular faces, 5 at each of the 12 vertices.
It is shown how to inscribe these solids in a given sphere, and how to determine the lengths of their edges.
§ 90. The 13th book, and therefore the Elements, conclude with the scholium, “that no other regular solid exists besides the five ones enumerated.”
The proof is very simple. Each face is a regular polygon, hence the angles of the faces at any vertex must be angles in equal regular polygons, must be together less than four right angles (XI. 21), and must be three or more in number. Each angle in a regular triangle equals two-thirds of one right angle. Hence it is possible to form a solid angle with three, four or five regular triangles or faces. These give the solid angles of the tetrahedron, the octahedron and the icosahedron. The angle in a square (the regular quadrilateral) equals one right angle. Hence three will form a solid angle, that of the cube, and four will not. The angle in the regular pentagon equals 65 of a right angle. Hence three of them equal 185 (i.e. less than 4) right angles, and form the solid angle of the dodecahedron. Three regular polygons of six or more sides cannot form a solid angle. Therefore no other regular solids are possible. (O. H.)
II. Projective Geometry
It is difficult, at the outset, to characterize projective geometry as compared with Euclidean. But a few examples will at least indicate the practical differences between the two.
In Euclid’s Elements almost all propositions refer to the magnitude of lines, angles, areas or volumes, and therefore to measurement. The statement that an angle is right, or that two straight lines are parallel, refers to measurement. On the other hand, the fact that a straight line does or does not cut a circle is independent of measurement, it being dependent only upon the mutual “position” of the line and the circle. This difference becomes clearer if we project any figure from one plane to another (see Projection). By this the length of lines, the magnitude of angles and areas, is altered, so that the projection, or shadow, of a square on a plane will not be a square; it will, however, be some quadrilateral. Again, the projection of a circle will not be a circle, but some other curve more or less resembling a circle. But one property may be stated at once—no straight line can cut the projection of a circle in more than two points, because no straight line can cut a circle in more than two points. There are, then, some properties of figures which do not alter by projection, whilst others do. To the latter belong nearly all properties relating to measurement, at least in the form in which they are generally given. The others are said to be projective properties, and their investigation forms the subject of projective geometry.
Different as are the kinds of properties investigated in the old and the new sciences, the methods followed differ in a still greater degree. In Euclid each proposition stands by itself; its connexion with others is never indicated; the leading ideas contained in its proof are not stated; general principles do not exist. In the modern methods, on the other hand, the greatest importance is attached to the leading thoughts which pervade the whole; and general principles, which bring whole groups of theorems under one aspect, are given rather than separate propositions. The whole tendency is towards generalization. A straight line is considered as given in its entirety, extending both ways to infinity, while Euclid never admits anything but finite quantities. The treatment of the infinite is in fact another fundamental difference between the two methods: Euclid avoids it; in modern geometry it is systematically introduced.
Of the different modern methods of geometry, we shall treat principally of the methods of projection and correspondence which have proved to be the most powerful. These have become independent of Euclidean Geometry, especially through the Geometrie der Lage of V. Staudt and the Ausdehnungslehre of Grassmann.
For the sake of brevity we shall presuppose a knowledge of Euclid’s Elements, although we shall use only a few of his propositions.
§ 1. Geometrical Elements. We consider space as filled with points, lines and planes, and these we call the elements out of which our figures are to be formed, calling any combination of these elements a “figure.”
By a line we mean a straight line in its entirety, extending both ways to infinity; and by a plane, a plane surface, extending in all directions to infinity.
We accept the three-dimensional space of experience—the space assumed by Euclid—which has for its properties (among others):—
Through any two points in space one and only one line may be drawn;
Through any three points which are not in a line, one and only one plane may be placed;
The intersection of two planes is a line;
A line which has two points in common with a plane lies in the plane, hence the intersection of a line and a plane is a single point; and
Three planes which do not meet in a line have one single point in common.
These results may be stated differently in the following form:—
| I. A plane is determined— | A point is determined— |
| 1. By three points which do not lie in a line; 2. By two intersecting lines; 3. By a line and a point which does not lie in it. |
1. By three planes which do not pass through a line; 2. By two intersecting lines 3. By a plane and a line which does not lie in it. |
| II. A line is determined— | |
| 1. By two points; | 2. By two planes. |
