| 3 | A straight line is at right angles to a plane when it makes right angles with all the straight lines which meet it and are in the plane. |
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| 4 | A plane is at right angles to a plane when the straight lines drawn in one of the planes at right angles to the intersection of the planes are at right angles to the remaining plane. |
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| 14 | When a semicircle with fixed diameter is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a sphere. |
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| 15 | The axis of the sphere is the straight line which remains fixed and about which the semicircle is turned. |
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| 16 | The center of the sphere is the same as that of the semicircle. |
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| 17 | A diameter of the sphere is any straight line drawn through the center and terminated in both directions by the surface of the sphere |
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