BOOK XI.
DEFINITIONS.
1. A solid is that which has length, breadth, and thickness.
2. That which bounds a solid is a superficies.
3. A straight line is perpendicular, or at right angles, to a plane, when it makes right angles with every straight line meeting it in that plane.
4. A plane is perpendicular to a plane, when the straight lines drawn in one of the planes perpendicular to the common section of the two planes, are perpendicular to the other plane.
5. The inclination of a straight line to a plane is the acute angle contained by that straight line, and another drawn from the point at which the first line meets the plane to the point at which a perpendicular to the plane drawn from any point of the first line above the plane, meets the same plane.
6. The inclination of a plane to a plane is the acute angle contained by two straight lines drawn from any the same point of their common section at right angles to it, one in one plane, and the other in the other plane.
7. Two planes are said to have the same or a like inclination to one another, which two other planes have, when the said angles of inclination are equal to one another.
8. Parallel planes are such as do not meet one another though produced.
