A right line is perpendicular to a plain, when it makes right The inclination of a right line to a plain, is the acute angle - - wo Book XI. - VI. - Jryov-The inclination of a plain to a plain, is the acute angle contained by the right lines drawn in both plains, to the same point of their common section, and making right angles with 1t. VII. Plains are inclined similarly, when their angles of inclination are equal. VIII. Parallel plains are such, which being produced, never meet. Similar solid figures are such as are contained under an equal number of fimilar plains. A solid angle is the inclination of more than two right lines that meet in one point, but are not in the same superficies. XI A pyramid is a solid figure, contained by more than two \ plains set upon one plain, and meeting at one point in the Vertex, - A prism is a solid figure contained by plains, whereof the two opposite are equal, fimilar, and parallel; and the other pa rallelograms. - tion. XV The axis of a sphere is that fixed right line about which the femicircle revolves. XVI. The diameter of a sphere is a right line drawn through the centre, and terminated on either side by the superficies of the sphere. XVIII, A cone is a solid figure described by a right angled triangle revolving about one of the fides, containing the right angle, remaining fixed. If the fixed right line be equal to the other side containing the right angle, then it is a rečtangular cone; |