224 
OF THE PARTS OF 
[sect. VII. 
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 w r ater 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.