When the gas reaches the pressure chamber, the spring, M, contracts, and consequently the counterpoise descends, and causes the cord, O, which carries the slide and writing style, to wind around the pulley. When the pressure diminishes, the movement takes place in an opposite direction. The tracing is done by means of a special form of style giving indelible curves through the medium of colored glycerine. The position of the point is determined in such a way as to annul the friction of the pen, and consequently to give it greater sensitiveness. It should be remarked that the course of the rod, K, is amplified in the tracing of the ordinates of the pressure according to the ratio of the diameters of the pulleys, N and O. The apparatus may be carried by hand by means of the handle, S, either in or out of its case. To put it in operation, it is only necessary to connect the apparatus with a gas burner (located near the place where the variations of pressure are to be observed) by means of rubber tubing. The apparatus may be employed under the same circumstances as glass and U-shaped water manometers, with the further advantage that the results are registered, and consequently can be more easily compared. * * * * * CENTRIFUGAL APPARATUS FOR CASTING METALS. The apparatus represented in Figs. 1, 2, 3, and 4 is the invention of Messrs. Taylor & Wailes, and is designed for casting metallic objects in annular form, its arrangement being slightly varied according to the nature of the objects to be cast. In all cases where a special form is to be given to the outer or inner circumference of the object, or where it is desired to exert a pressure on the circumference, such form or pressure is obtained by the introduction of a core which may be expanded or contracted as need may be. For this purpose an expansible, metallic core is employed, the arrangement of which is shown in Figs. 1 and 2, and which is so fashioned that the inner circumference of the ring to be cast may receive the desired form. This core is formed of the pieces, g, g', made of cast-iron or any other material which fuses with difficulty, and which are placed in the revolving mould in such a way that after the cooling of the pieces the parts, g, recede by the shrinkage of the piece and thus free the core. The parts, g, of the core are in the shape of circular segments, and are united at their external circumference by a flange, along with which they form a shoulder piece for the casting. As a consequence of the rapid revolution of the mould, these parts are pressed by centrifugal force against the molten metal which is run into the mould. [Illustration: CENTRIFUGAL METAL MOULDING APPARATUS.]