Part Weight lb. oz. I. Body of the top-- Mean diameter of ring, 4 inches. Section of ring, $\frac{1}{3}$ inch square. The conical portion rises from the upper and inner edge of the ring, a height of $1\frac{1}{2}$ inches from the base. The whole body of the top weighs 1 7 Each of the nine adjusting screws has its screw 1 inch long, and the screw and head together weigh 1 ounce. The whole weigh 9 II. Axle, &c.-- Length of axle 5 inches, of which $\frac{1}{2}$ inch at the bottom is occupied by the steel point, $3\frac{1}{2}$ inches are brass with a good screw turned on it, and the remaining inch is of steel, with a sharp point at the top. The whole weighs $1\frac{1}{2}$ The bob $B$ has a diameter of 1.4 inches, and a thickness of .4. It weighs $2\frac{3}{4}$ The nuts $b$ and $c$, for clamping the bob and the body of the top on the axle, each weigh $\frac{1}{2}$ oz. 1 Weight of whole top 2 $5\frac{1}{4}$ The best arrangement, for general observations, is to have the disc of card divided into four quadrants, coloured with vermilion, chrome yellow, emerald green, and ultramarine. These are bright colours, and, if the vermilion is good, they combine into a grayish tint when the rotation is about the axle, and burst into brilliant colours when the axis is disturbed. It is useful to have some concentric circles, drawn with ink, over the colours, and about 12 radii drawn in strong pencil lines. It is easy to distinguish the ink from the pencil lines, as they cross the invariable axis, by their want of lustre. In this way, the path of the invariable axis may be identified with great accuracy, and compared with theory. * 7th May 1857. The paragraphs marked thus have been rewritten since the paper was read.