Definition 17.—A solid having any plane figure for its base, and its sides plain triangles terminating in the same point, is called a pyramid.
Definition 18.—A solid having a circle for its base, and terminating in a poitt, such that a straight line extended from any part of the circumference of the base to the terminating point may be in the surface of the solid, is called a cone ; and the surface which lies between the circumference of the base and the terminating point, is called the conic surface.
Definition 19.-1f the plane of a circle be supposed per pendicular to a given plane, with its circumference or edge upon that plane ; and if there be a straight line standing on any other point perpendicular to the said plane ; and if another straight line be made to move parallel to the plane on which the circle stands, so as always to touch the circum ference and the straight line, beginning at any given point, and proceeding entirely round until it arrives at the same point ; then the solid bounded by the circle, and the surface passed over by the straight line contained between the circumference of the circle and the straight line is called a conoid; and the surface generated by the straight line is called a conoidol surface.
Definition 20.—A sphere is a solid formed by the revolu tion of a semicircle upon its diameter.
Definition centre of a sphere is the same with that of the semicircle.
Definition diameter of a sphere is any straight line which passes through the centre, and is terminated both ways by the superfieies of the sphere.
Definition 23.--A cube is a solid figure contained by six equal squares.
Definition 24.—A tetrahedron is a solid figure contained by four equal and equilateral triangles.
Definition 25.—An octahedron is a solid contained by eight equal and equilateral triangles.
Definition 26.—A dodecahedron is a solid contained by twelve equal pentagons, which are equilateral and equi. angular.
Definition 27.-11n icosahedron is a solid contained by twenty equal and equilateral triangles.
The solids defined in the last five definitions are called the five regular solids.