Land and Marine Surveying (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1868 Excerpt: ...turn the vernier plate round to bear on the pole held at station 5 3 tighten clamp c, and perfect the bisection of the pole by means of the tangent screw T. Read off the angle. The verniers of the instrument are at station 4, in the same position relative to the magnetic, as line 5,1, in fig. 49, as shown by the vernier of the protractor. The line 5, 4, is parallel to the line 5, 1. It will be observed that at every remove of the instrument the first thing to do after levelling the instrument, is to loosen the lower clamp--that which is beneath the parallel plates--and to bring the instrument to bear on the back station, after having reversed the telescope; this perfected, we apply ourselves to the vernier clamp c, which loosens the vernier plate, enables us to turn it round, and obtain the required angles. In this manner, above described, we complete the circuit of our survey, repeating the same operations; we need not therefore refer to them any further at present. Let it however be observed that on setting up the instrument at station 9, we have completed the visual circuit. Now the first thing we have again to do here, is to loosen the lower clamp, reverse the telescope, turn round to the pole at station 8, perfect the bisection, release the vernier plate by loosening the screw c; reverse the telescope, turn round to station 1, tighten the clamp c, and perfect the bisection. Head off the angle. We have now the critical moment of the survey, because the vernier should now read precisely the same angle as it did when at station 1 we took the bearing of the line 1, 9. If this is exact, then the work is very correct, and if the chaining has been good the work will be perfect. It is seldom, however, this occurs so closely as we could wish, and there is often...