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The characteristic property of all cylindrical surfaces is that the tangent planes are parallel to the axis.

If we revolve this about the line of the radius in question, as an axis, the circle will generate the sphere again, and the tangent line will generate the tangent plane.

A cone of second order is uniquely determined by five of its edges or by five of its tangent planes, or by four edges and the tangent plane at one of them, &c. &c.

The direction of frictional resisting forces is in the common tangent plane of the two surfaces, and directly opposed to their relative motion.

Considered as a curved surface, concentric with the earth, a tangent plane to such a surface is the plane of the horizon.