Transactions . s, in Buch a athe same operations must be repeated at each station. It is i to see that a slight displacement of the instrument couldnot, under the circumstances, be de-tected, and that, in Buch a case, theling of a vertical angle would bevitiated. In no case Bhould the accu-racy of the vertical line passing throughthe vernier-zero be referred to theshort levels inside the compass-box,supposing that such exist. The ver-nier-bar may not be so nicely fixed thatit- zero would be in the perpendicularline when the spirit-bubbles in the com-pass-box arc supposed to be level. The verni
Transactions . s, in Buch a athe same operations must be repeated at each station. It is i to see that a slight displacement of the instrument couldnot, under the circumstances, be de-tected, and that, in Buch a case, theling of a vertical angle would bevitiated. In no case Bhould the accu-racy of the vertical line passing throughthe vernier-zero be referred to theshort levels inside the compass-box,supposing that such exist. The ver-nier-bar may not be so nicely fixed thatit- zero would be in the perpendicularline when the spirit-bubbles in the com-pass-box arc supposed to be level. The vernier-bar of the vertical circlein Mr. Stanleys instrument, as in allgood instruments, should therefore have its own attached inde-pendent spirit-level, in order to determine the perpendicularitywhen the telescope is in movement: besides, it offers greatereconomy in time and facility in working. Conical or Sloping Graduations.—Mr. Lyman, admitting thesuperior convenience of such graduation to the observer,Bays :. Vertical Circle, with Spirit-Level Attached. But the sloping graduations are disliked by instrument-makers, becausemore troublesome from the increased difficulty of correcting the centering andof getting tbe vernier precise: and therefore repairs are much more expen-sive. This mav be the case anions the American makers with * Trans., xxxi., 91. 722 REMARKS I PON SURVEYING INSTRUMENTS. whom Mr. Lyman is acquainted, bul it cannot be suid of thebest European instrument-makers, especially those of greatestrepute in England. The writer is fully aware thai circles wereformerly divided upon the level surface or flat, for surveying-purposes, and this practice was continued for a considerabletime; but after a good deal of experience, engineers and sur-veyors did not like the plan, and, consequently, the conical orsloping divisions were introduced, and have been continued fora great number of years. When, however, the circles arelarge, as in some of Troughton & Simmss 36-in. inst
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Keywords: ., bo, bookcentury1800, bookdecade1870, booksubjectmineralindustries
