So large a proportion of the motions of the parts of machinery may be included in ths classes rectilinear and circular, that the very few exceptions wherein the curvilinear motions are other than ...

If lines are perpendicular to each other then \({m_1} \times {m_2} = - 1\) conversely if \({m_1} \times {m_2} = - 1\) then the lines are perpendicular to each other.

Two non-parallel lines will have a common point -the point of intersection - where they cross each other or meet.