FORCE-CLOSURE AT DEAD POINTS. 187
to a may be applied at the pin 2, and by its means the slider c will be continually driven backwards ard forwards in the slot d, by the connecting-rod &. If, however, the driving-force be applied to c instead of to a, so as to cause it to reciprocate between 3' and 3", there is nothing to insure the continuance of the motion beyond the positions 3' or 3" of the block, and 2' or 2" of the crank pin, for in these positions the driving effort passes through the axis of the fixed bearing 1, and is therefore received direct by the stationary link d. These two positions form what are called the "dead points " of the mechanism. The fundamental idea of this expres- sion is that the chain opposes itself as a rigid body fixed to its stationary link, to the action of the driving force ; that its move- ability, its life, is lost, it lies as it were dead. This idea we shall generalise by applying it, not merely to the crank mechanisms, for which Watt first used it, but to all other mechanisms in which a similar condition occurs.
��FIG. 145.
Several means are employed for carrying a mechanism over its dead points. In the case of Fig. 145, and others similar to it, it is usual to employ a revolving mass, connected with some link of the chain which is moving with suitable velocity when the driven link is at its dead points, and to use a portion of the energy stored in such a mass to carry the mechanism over the dead point. In the application of this train to the steam-engine, everyone is familiar with the fly-wheel, rigidly connected with the crank a, which is so often used for passing the dead points. The fly- wheel furnishes the sensible force required to continue the motion of the machine; the continued motion of the chain is therefore effected by force-closure. In the locomotive, so soon as the train is once set in motion, the whole mass, moving directly onward, serves the same purpose as a fly-wheel ; but in general rotating bodies are used. Now and then we find simple weights
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