178 KIN EM A TICS OF MA CEINER Y.
incomplete pairs of elements and are much used for that purpose, as in the case of spring-packed pistons, spring pawls for ratchet- wheels, etc.; they play also a most important part in the storing of energy, to which we shall return further on. Springs of organic mate- rial, india-rubber, vegetable fibre, skin, etc. greatly resemble tension- organs ; those of hard material seem more like the rigid elements. In their mode of action they rather resemble the pressure-organs, as being, within somewhat wide limits, elastic. They differ essentially from the rigid elements, however, notwithstanding the apparent resemblance, for in these the flexibility is supposed to be reduced to so small an amount that it may be neglected, while in the springs it is intentionally made very considerable.
�43.
Closure of a Pair of Elements by a Kinematic Chain.
An incomplete pair of elements may also be closed by kinematic linkage. Two bodies a and b (Fig. 133) having for their axoids circular cylinders, and having their surfaces fluted, in other words,
two spur-wheels furnished with accurately fitting teeth, have by their profile forms the necessary restraint in the { direction of the tangent T T ; the teeth may also be so made that the wheels cannot be moved nearer together ; there is required only restraint against divergent motion in the direction N N 1 the nor- mals. This can be supplied by one of the methods considered in a former article. Let us take the fifth method ( 35), in which parallels to roulettes serve as profiles, then we have as a parallel to the (point-)path of the centre of one of the wheels a circular ring, and as a parallel to the other centre itself a circle, these giving us the profile shown in Fig. 134, an annular groove for the wheel a, and a cylindric pin moving in it for the wheel b } and by these the
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