Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/394

 in the direction of the margin. Wood is preferred to metal for the feet, as less likely to slip on a smooth surface; and they are made broad at the end, to prevent the position of the plate and the firmness of its rest from being too much affected by the minute inequalities of a rough one. The flat ends of the feet are adjusted to a plane parallel to the upper surface of the plate; and the fibre of the wood is set perpendicularly, in order to prevent any derangement from the expansion and contraction of the wood attending the changes of moisture. On the upper surface near the edge, is a circle divided into degrees, which are numbered from zero to 90° through the four quarters successively in the same order; and it is of no consequence whether this is done from left to right, or from right to left. In the center of the plate is a circular hole b, to receive an axis on the under side of the base of the quadrant. For securing it, there is a bolt c c, which slides below, and is worked by a catch d rising through an oval hole in the plate and made Hush with its surface.

B represents the quadrant, which has all its frame-work of brass. CC is its base, which traverses upon the plate; at e is the button of the central axis, which is secured in the center of the plate by the hollowed end of the bolt beneath, and at f is the clamp and milled-head screw, for fixing the quadrant to the edge of the plate in any direction. At l is a rectangular aperture, through which the divisions of the plate are seen, and read off by an index on the middle of the bevelled side of the aperture nearest to the central axis. DD is the graduated arch, fixed firmly to the base, and at right angles to the plane of its under surface. It is divided into degrees, which are numbered from below. At b is the head of a short axis passing through the center of the arch, and revolving with the motion of the radial bar m m, which carries with it the