SECT, vii.] STEAM ENGINES. 217 



which unspun long hemp, or soft rope prepared for the purpose, and called gasket, 

 is wound as evenly and compactly as possible, to form the packing. This packing 

 is compressed together by a plate or cover C, which is put over the top of the 

 piston, having a projecting ring to fit over the lower part, and complete the upper 

 side of the space for the packing, the pressure being produced by screws S S, &c. 

 Both the upper and lower part of the space round the piston, to contain the 

 packing, is a little curved, that the pressure produced by the screws on the 

 packing may force it against the inside surface of the cylinder, into as close 

 contact as possible. 



The screws being tightened when the piston is in the cylinder, the particular 

 form of the piston has the effect of squeezing out the packing, and causing it to 

 press forcibly against the inside of the cylinder at its upper and lower edges. 

 When the packing wears so as to become too small by use, these screws, which 

 are more or less in number according to the size of the piston, are always resorted 

 to for tightening it, as long as they are capable of acting ; and when this is no 

 longer the case, the piston top must be removed, and an additional quantity of 

 new packing introduced. The piston rod is generally attached to the bottom part 

 of the piston, by passing it upwards into a conical hole made to receive it, to 

 which the bottom of the rod is exactly fitted, and a screw nut, or a wedge, between 

 the top and bottom is inserted, which effectually secures it. 



The piston is kept supplied with melted tallow by means of a funnel on the 

 top of the cylinder lid, provided with a cock to prevent the escape of steam. 



468. WOOLF'S PISTON. In the usual method, whenever the piston, by continued 

 working, becomes too small and occasions a waste of steam, it is necessary to take 

 off the top of the cylinder, in order to get at the screws, even when fresh packing 

 is not wanted. This being laborious work, is therefore generally avoided by the 

 person who attends the engine, as long as it can possibly be made to work without 

 taking this trouble ; and the neglect occasions a great and unnecessary waste of 

 steam, and consequently of fuel in proportion. 



The object of Mr. Woolf s improvement is to enable the engine man to tighten 

 the piston, without the necessity of taking off the cover of the cylinder, except 

 when new packing becomes necessary. He accomplishes this by the following 

 methods. 



To the head of each of the screws a small toothed wheel is fixed, so that it may 

 be turned, and therefore tightened, by means of a central toothed wheel, which 

 works upon the piston rod as an axis : if one of the small wheels be turned, it 

 turns the central wheel, and the latter turns the others. The one which is to be 

 turned by the handle is furnished with a projecting square head, which rises up 



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