224 OF THE PARTS OF [SECT. vn. 



The middle groove R is formed between the two others to receive grease ; and a 

 circular cavity S S is also made around the hole in the cover of the cylinder for 

 the same purpose. 1 



In constructing this collar the blocks L should be parallel, otherwise the wear 

 will be irregular, and the springs will soon be ineffective ; the blocks M should not 

 wear unless the piston rod wears ; and perhaps it will be steam-tight, unless 

 assisted by a hemp packing behind the hoops N N. 



MODES OF OPENING VALVES, COCKS, AND SLIDES. 



478. The motion may be given either from the reciprocating or from the 

 rotary parts of an engine. In engines which have no rotary parts, motion is 

 communicated to the valves by a rod or beam, called a PLUG TREE, attached to the 

 engine beam near to the end moved by the piston rod. This plug tree is provided 

 with certain adjustible projections called TAPPETS, which strike the levers or handles 

 of the valves, and thus open and shut them at the proper intervals as the beam 

 ascends or descends. These handles turn on axes, and act as levers to move the 

 valves, slides, or cocks. The most important point is to render the action certain, 

 for the effect of the engine depends on the passages being opened and closed at 

 the proper times. When valves are employed, they are generally opened by 

 weights. (See Plate ix. Fig. 3.) A weight w, sufficient to overcome the friction 

 and open the valve, acts by a short arm a on the axis, which requires to be turned 

 to move the valve ; the weight is kept suspended by a spring catch b while the 

 valve is close, and when the catch is disengaged by the handle c, being moved by 

 the tappet d, the valve opens. If the valve be large, it requires a considerable 

 weight w to open it against the pressure of the steam ; and in that case either the 

 valve described in art. 442, or Watt's mode of relieving the pressure, may be 

 adopted. It will naturally be inquired, why weights are raised to open the valves 

 instead of using the direct power of the beam. The only reason assigned for so 

 doing is, that a weight opens a valve more rapidly, and the loss by closing them 

 slowly was not quite so readily detected ; though the absolute loss is about 

 the same, and the practice is becoming more common to open them by direct 

 action. 



The descent of the weight which opens a valve is regulated by an ingenious 

 method: it either descends into, or forces a piston into a vessel of water, (see 

 C, Fig. 3. Plate ix.) while the aperture by which the water escapes from under 

 it may be increased or diminished at pleasure ; the weight therefore acts with its 



1 Gill's Technical Repository, vol. iv. p. 242. 



