The process of removing the top of the cylinder for the purpose of tightening the screws in the piston is one of so laborious a nature, that the men entrusted with the superintendence of these machines are tempted to allow the engine to work notwithstanding injurious leakage at the piston, rather than incur the labour of tightening the screws as often as it is necessary to do so. To avoid this inconvenience, the following method of [Pg244] tightening the packing of the piston without removing the lid of the cylinder, was contrived by Woolf. The head of each of the screws was formed into a toothed pinion, and as these screws were placed at equal distances from the centre of the piston, these several pinions were driven by a large toothed wheel, revolving on the piston-rod as an axis. By such an arrangement it is evident that if any one of the screws be turned, a like motion will be imparted to all the others through the medium of the large central wheel. Woolf accordingly formed, on the head of one of the screws, a square end. When the piston was brought to the top of the cylinder, this square end entered an aperture made in the under side of the cover of the cylinder. This aperture was covered by a small circular piece screwed into the top of the cylinder, which was capable of being removed so as to render the square head of the screw accessible. When this was done, a proper key being applied to the square head of the screw, it was turned; and by being turned, all the other screws were in like manner moved. In this way, instead of having to remove the cover of the cylinder, which in large cylinders was attended with great labour and loss of time, the packing was tightened by merely unscrewing a piece in the top of the cylinder not much greater in magnitude than the head of one of the screws. This method was further simplified by causing the great circular wheel already described to move upon the piston-rod, not as an axis, but as a screw, the thread being cut upon a part of the piston-rod which worked in a corresponding female screw cut upon the central plate. By such means, the screw whose head was let into the cover of the cylinder which turned, would cause this circular plate to be pressed downwards by the force of the screw constructed on the piston-rod. This circular plate thus pressed downwards, acted upon pins or plugs which pressed together the top and bottom of the cylinder in the same manner as they were pressed together by the screws connecting them as already described. METALLIC PISTONS.