To set out a tangent to the curve at A, Fig. 3, set up a rod at A and
another at any point C, and take up a position on the curve at some
point between them. Adjust the mirror until the rods are seen
superimposed. Then moving back to A, observe C direct, and set up a
rod at E in the line observed by reflection. Then A E is the tangent
required. Similarly, on completing the setting out of a curve, and
arriving at the end of the chord, the remote end being seen by
reflection, the direction observed along the axis of the eyepiece is
the new tangent.
Any of the angles or other ratios already mentioned may be used for
setting the instrument, but if no data whatever are given, as in the
rough surveys for colonial railways where no previous surveys exist,
it is only necessary to select points through which the curve must
pass, to set up ranging rods either at the extremities of the desired
curve, or at any points thereon, to take up a position on the desired
curve between two rods, and to adjust the instrument until they are
seen in coincidence.
Pages:
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76