Book 11
10
Equal and similar solid figures are those contained by similar planes equal in multitude and in magnitude.
11
11
A solid angle is the inclination constituted by more than two lines which meet one another and are not in the same surface, towards all the lines.
11
Otherwise: A solid angle is that which is contained by more than two plane angles which are not in the same plane and are constructed to one point.
12
12
A pyramid is a solid figure, contained by planes, which is constructed from one plane to one point.
13
13
A prism is a solid figure contained by planes two of which, namely those which are opposite, are equal, similar and parallel, while the rest are parallelograms.
14
14
When, the diameter of a semicircle remaining fixed, the semicircle is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a sphere.
15
15
The axis of the sphere is the straight line which remains fixed and about which the semicircle is turned.
16
16
The centre of the sphere is the same as that of the semicircle.
17
17
A diameter of the sphere is any straight line drawn through the centre and terminated in both directions by the surface of the sphere.
18
18
When, one side of those about the right angle in a right-angled triangle remaining fixed, the triangle is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a cone.
