Helix, or Spiral, is a curve of much interest in mathematics. It is traced out by a moving point that combines a rotation about an axis with a rectilinear movement in a line parallel to the axis. An ordinary screw-thread affords an example of the regular helix. The amount of onward motion in the direction of the axis that is produced in one complete rotation is called the pitch. Thus if a screw of -J-inch pitch, penetrating a piece of wood, is given one complete revolution it advances 4; inch, and proportionately less for any fraction of a revolution. This uniformity of advance shows that the spiral can be produced by wrapping a triangular slip of paper round a cylinder.