188
�KINEMATICS OF MACHINERY.
�which act statically, and not as reservoirs of actual energy, in
effecting the passage of the dead points by force-closure.
�46.
Passage of the Dead Points by Chain-closure.
The use of force-closure in passing the dead points is not suit- able in every case. In the steam-engine especially it has frequently been necessary to resort to another principle. In this case the dead point has to be crossed repeatedly when a portion of the mechanism has a very small velocity, is just passing from rest to motion, as is the case in locomotives, marine-engines, winding engines, etc. This other principle consists in the employment of a second kinematic chain so connected with the first that it is always
��FIG. 146.
in an advantageous position for action when the first is dead. In most cases the two chains are similar, so that their action on each other is reciprocal.
In order to close the above crank train, for instance, at its dead points, it is so combined with a second similar mechanism (Fig. 146) that the two cranks have a common axis and are 90 apart, the directions of the guides being parallel. This form of mechanism has of course most extensive application to locomotives and double-cylinder engines generally. If instead of using cranks
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