CAM CHAINS. 563
can take. We must mention here, however, the use of a pressure- organ in the chain. The chamber-wheel trains of Chap. XL belong to the compound chains ; we have, however, simple chains in which a fluid -that is a pressure-organ takes the place of one of the wheels, and the pairing of the fluid with its chambei takes the place of one of the cylinder pairs. Among these are the common water-wheel, the lift-wheel, and the paddle- wheel (cf. 61 and 62), and also some turbines and centrifugal pumps.
8 157. Cam Chains.
We noticed the cam trains very briefly in 120, and recognized the desirability of their separate treatment. Fig. 413 represents one of these trains. Its formula is (C^C,^) ; we have already examined (p. 537) the nature of the pair- ing between the earn and the click or tooth. The special forms which this chain can take are very numerous. The cam chain, however, is not here represented in anything like its highest form. The latter* so far as is conditioned by the form of FlG
the cam and the tooth, would be the chain
'(C\r,~), which is formed from two pairs (R) = (0) of the first order, and the highest forms of the pairs of the fourth order ( 146). The most general form of all will be obtained if we substitute (as in 155) the higher pairs (&,) or (K>) for (0), a method which also leads to the highest forms of the simple spur-wheel chains.
Under this most general form of the cam chain there come those important special cases which we have called click-trains. We obtain these by using in the chain pairs of orders VI. or VI I. ( 146). We thus obtain the chains :
(C' 2 # z ;) and (CJtfJ)
with their numberless simpler forms. As we have studied several typical cases of these in Chapter XT. ; we might now leave them
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