How to Run a Line through a Given Point Parallel to a Chain Line.

To run a Line through a Given Point Parallel to a Chain Line.

  Let AB, fig. 22, be the chain line and C the point through which it is desired to lay out a line parallel to AB.

  From C lay out CD perpendicular to AB and measure the length of CD. Choose a point E on the line AB as far as possible from D and at E erect a perpendicular EF to AB equal in length to CD. The points C and F will then be on a straight line parallel to AB.

  An alternative method is to choose points D and E, fig. 23, suitably placed on the line AB and measure the distance CD, leaving arrows at every chain length near the point which appears to be the centre of the line. Having obtained the length of CD, use the arrows to put a mark at F so that CF = FD. Measure the distance EF and prolong the line to G so that FG = EF. Then the line CG is parallel to AB.

  In the above case the point C is supposed to be accessible. If it is inaccessible, as in fig. 24, establish the foot D of the perpendicular CD on the line AB by the method described on previous tropic and obtain the distance DC by one of the methods described on next. A point F on the line CF can now be obtained by laying out EF perpendicular to AB and making EF equal in length to DC.