542 KINEMATICS OF MACHINERY.
148. Pairs of Elements containing Pressure-Organs.
"We Lave seen ( 56) that the pressure-organ Q takes several special forms : the liquid Q^ the gaseous Q^ and the globular or grained Q~ or Q . Although these forms have very important points of difference, yet so far as their kinematic pairing is concerned they may all be denoted by the single symbol Q.
The cha,racteristic of the element Q that it has no resistance except to compression, allows it to be paired with rigid elements in the most various ways. It can be used as one of the elements in all the first five orders of pairs mentioned in 146, as a substitute for a rigid element. The turbine, the screw-propeller, the water- wheel, the chamber-wheel train, the pug-mill and so on, give us numerous and various examples of such pairs. We have therefore the orders
�The order (H~>^ and its lower form (-H.J may also be distinguished,
but it is more convenient to include both in the order (> jq ).
We have already found, further, that pressure-organs are paired both in free and in fast click-trains, in those namely, in which the clicks are valves. If we consider the valve in such pairs as a tooth, Z, (which the analogy of the rigid click-trains allows us readily to do), we can indicate the two orders of pairs thus obtained by the symbols
(C) and (ft.:).
In none of the cases mentioned is the pairing possible without another pairing taking place at the same time, that namely of the pressure-organ with its vessel or chamber, F~, Fig. 378. Besides this we have also the pairing with the piston F + (Fig. 379), so that we may write this order of pairs in general as
�We have noticed before ( 41) that this kind of pairing can
be and has been also extended to the tension-organs, as in the link-
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