554 KINEMATICS OF MACHINERY.
it in a form with which we are now familiar, Fig. 398 shows it in another form, in which all three prism pairs are closed. If instead of three we attempt to combine two prism pairs into a chain we
��FIG. 397.
�FIG. 398.
�obtain either a single pair or a fixed chain. From four prism pairs,
however, we can very easily form a simple chain, as is shown in
Fig. 399, of which the formula is (P-). We might treat the chain
��FIG. 399.
�as derived from this one by making the angle between the
pairs 3 and 4 infinitely small. The chain (P^-) itself, however,
might be considered as obtained from (OJ[) by making all four
links infinitely long. But closer examination shows that the chain
(P^-) is not constrainedly closed. If we suppose, for example,
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