The arms to which the board is fastened rest upon the sloping upper face of a rather ?at hollow cone of brass, to which they are permanently fixed. Upon its lower edge or periphery this cone is fashioned into a horizontally projecting rim, the inferior face of which is as nearly as possible a perfect plane, and this in its turn rests upon a corresponding rim of somewhat greater diameter projecting slightly beyond it. This second rim forms the upper and outer ?ange of a circular metal disk in the form'of a very Shallow cylinder. The inferior face or plane of the upper ?ange or rim has, at its contact with the superior face of the lower, a horizontal rotary movement about a common center which is also the center of the instrument, and the two are, held together by means of a solid conical axis of brass extending upward from the center of the inner face of the lower disk. A socket of similar shape fits exactly over this axis, projecting downward from the inner Side of the apex of the conical or upper disk. The two plates are held together by means of a screw with a milled head, capping the cone from the outside, and which can be loosened or removed at pleasure. A tangent screw and clamp fastened to the edge of the upper rim permit, when loose, the revolution of the table about its center, and, when clamped to the lower limb, hold the table firm while the tangent screw gives a more delicate movement. Three equidistant vertical projections of brass, grooved on the underside, and cast in one piece with the under face of the lower disk, extending from the periphery toward the center, rest upon the points of three large screws which come through a heavy wooden block below. This block, which is the top of the stand and is approximate in form to an equilateral triangle, is 254 inches thick when made of wood. It is contemplated having the board made of a special aluminum alloy.