THE STEAM-ENGINE. 



485 



this will force out the packing and render the piston steam-tight. This pack- 

 ing is lubricated by melted tallow let down upon the piston from the funnel 

 inserted in the top of the cylinder, furnished with a stop-cock to prevent the 

 escape of steam. The lower end of the piston-rod is formed slightly conical, 

 the thickest part of the cone being downward. It is passed up through the 

 piston, and a nut or wedge between the top and bottom is inserted so as to 

 secure the piston in its position upon the rod. 



The process of removing the top of the cylinder for the purpose of tighten- 

 ing the screws in the piston is one of so laborious a nature, that the men in- 

 trusted with the superintendence of these machines are tempted to allow the 

 engine to work, notwithstanding injurious leakage at the piston, rather than 

 incur the labor of tightening the screws as often as it is necessary to do so. 



To avoid this inconvenience, the following method of tightening the pack- 

 ing 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 mot^n will be imparted to all the others through 

 th^ 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 re- 

 moved 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 labor 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 al- 

 ready 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 corre- 

 sponding 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 downward by the force of the screw construct- 

 ed on the piston-rod. This circular plate thus pressed downward, 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. 



The notion of constructing a piston so as to move steam-tight in the cylin- 

 der without the use of packing of vegetable matter was first suggested by the 

 Rev. Mr. Cartwright, a gentleman well known for other mechanical inventions. 

 A patent was granted in 1797 for a new form of steam-engine, in which he 

 proposed to use the vapor of alcohol to work the piston instead of the steam of 

 water : and since the principle of the engine excluded the use of lubrication 

 by oil or tallow, he substituted a piston formed of metallic rings pressed against 

 the surface of the cylinder by springs, so as to be maintained in steam-tight 

 contact with it, independently either of packing or lubrication. Although the 

 engine for which this form of piston was intended never came into practical 



