CHAIN- AND FORCE-CLOSURE. 181
The hydraulic press gives us an instructive, example of a me- chanism combining chain- and force-closure. In Fig. 139 the pump valves are omitted for the sake of clearness. The vessel d carries the chain-closure between the pistons, a and b, with which it is prismatically* paired. At the same time it encloses the fluid c, which is force-closed by the pressures upon the two pistons in the only direction of motion left possible to it. The hydraulic press forms the contra-positive of a machine apparently most unlike it, the pulley -tackle, the pressure-organ water in the one being replaced by the tension-organ rope in the other. If we substitute rigid rounded bars at the top and bottom of the tackle for the usual pulleys, as in Fig. 140, the logical correspondence
��FIG. 140.
�between these two mechanisms becomes even more obvious.f The
chain now, indeed, has three links only, but this is merely because
the tension-organ does not need a confining vessel. The motion
of the piece &, as a whole, is still force-closed by the load.
If we used a prism pair to compel b to move in a straight line in
reference to a, the similarity would be still more complete. It is
worth noticing how many illustrations occur in modern engineering
of what we may call the interchangeability of tension- and pressure-
organs, of which we have had here an illustration. The arrange-
ment for ringing bells by air-pressure, now becoming extensively
- Each piston, although cylindric, forms a sliding pair with its stuffing-box,
turning being prevented by some external means.
t An error is frequently made which stands greatly in the way of understanding this matter, that namely of supposing the action of the tackle to be absolutely connected with the rotating block, or pulley, which in reality has no other object than that of lessening destructive friction. In anatomy the trachlea surface over which a tendon slides is rightly called a Rolle or pulley. R.
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