20. The base of a cone is the circle described by that side containing the right angle which revolves.
21. A cylinder is a solid figure described by the revolution of a right-angled parallelogram about one of its sides which remains fixed.
22. The axis of a cylinder is the fixed straight line about which the parallelogram revolves.
23. The bases of a cylinder are the circles described by the two revolving opposite sides of the parallelogram.
24. Similar cones and cylinders are those which have their axes and the diameters of their bases proportionals.
25. A cube is a solid figure contained by six equal squares.
26. A tetrahedron is a solid figure contained by four equal and equilateral triangles.
27. An octahedron is a solid figure contained by eight equal and equilateral triangles.
28. A dodecahedron is a solid figure contained by twelve equal pentagons which are equilateral and equiangular.
29. An icosahedron is a solid figure contained by twenty equal and equilateral triangles.
A. A parallelepiped is a solid figure contained by six quadrilateral figures, of which every opposite two arc parallel.
PROPOSITION 1. THEOREM.
One part of a straight line cannot he in a plane, and another other part without it.
If it be possible, let AB, part of the straight line ABC, be in a plane, and the part BC without it.
