COMBINED AND MIXED CHAINS. 571
of parallel crank- trains, it consists therefore of two chains of the form (6'g II <7J) having their d links equal and common, and their links a and c combined into ternary links. We know that this chain 2 (G || 6 2 ') has the property that both its parallel cranks can pass their dead points, which are also change-points (cf. 46 and 66) without stoppage or change of motion. This property, however, is characteristic of tire combination, neither chain by itself
��FIG. 426.
possesses it, the compounding has therefore given us in this case something new.
The anti-parallel cranks, Fig. 428, give us another illustration of the same thing. The object of our examination of this train in 47 and 67 did not lead us to notice that here also, although the number of links is not increased, we have a compound chain. It consists of our well-known four links a, b, c, d and a second chain having for its links, A\ B, (74 Z>, and d. The latter may be written in full
(t C <l
si+ ( r /\ t (*\ it n~
C+ ... (I ... (&) ... |! ... (V) ... j| ... (y w
Its frame d is identical with the link d of the chain (G") ; the two links C. ..||...Z coincide with the links a and e, both are therefore made ternary links. By themselves neither of the chains could move continuously, (C") would be stopped at .the dead points, (C"Z") is unclosed in every other position.
. These illustrations will suffice to show the difference between the two classes of compound chains. We shall call the class
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