Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/395

 spirit level EE, and the compass o o o. This axis is made to move with a degree of friction sufficient to support the weight of the radial bar in any position. The level turns upon its axis by two pivots p p set in two small plates projecting from the extremities of the radial bar, and through the same plates, immediately below the level, passes a strong conical axis r r, to one end of which the compass is fixed, and with which it revolves.

The compass-box has its bottom, as well as its top, made of plate-glass, and each of the two pieces is set in a cylindrical rim of brass which forms its sides. To the middle of this rim there is fixed, on the inside, an annular brass plate, silvered and graduated in the same manner both above and below. The divisions are of two degrees. There are two zeros, one in each intersection of a line passing through the center, and on both sides of each of them the divisions are numbered alike, till they meet in two nineties, in a line at right angles to the first. At g is a catch for working 4 bolt sliding on the outside, which lifts the needle from its pivot in the usual way, by the bent lever s. The needle has the north pole marked on the under, as well as the upper, side, and also an index line drawn from one end to the other.

In adjusting the different parts of the quadrant to each other regard must be paid to the following circumstances; that the axis of the level may be parallel to the line from the center of the quadrantal arch to the index on the radial bar; again, that when the radial bar is home upon the base, the axis of the level may be parallel to a line, in the plane of the base, from the center to the index of the base, and that the index of the radial bar may at the same time point to zero; also that the line of the two zeros in the compass may be parallel to the axis of the level, at least, when the plane of the compass is at right angles to that of the quadrantal arch.