TENSION-ORGAN PAIRS. 539
H = H and K = K may always be considered as falling into the divisions given above. Some cases which are, apparently, extremely complicated as, for example, the pairings occurring in Kose-engines, which are often constructed in the freest form, belong to the second order. Even the first order includes necessarily many quite free-forms. Although the number of primary divisions which we have employed is so small, we include among them, so as to make the classification as useful practically as possible, several which in strictness are only varieties of other orders, as, for instance, Nos. VI. and VII.
�147.
Pairs of Elements containing Tension-organs.
The characteristics of the tension-organs, band T p , rope '1\, wire, T e , chain T z or T n so far as their manner of pairing goes, i uiy be sufficiently indicated by the use of the symbol T for the whole of them, and this accordingly will suffice us here. We have seen that the pairing of T with other elements can take place only
��FIG. 372,
�under a tensile force-closure. It can be laid upon, or wound round,
rigid elements, but obviously only upon " positive " (see 56)
elements, so that its pairing with rigid elements is restricted to a
certain class of forms.
Beginning with the screw, we find both higher and lower forms of the pairing of T with S frequently in use. The common chain drum of a crane forms an element of the pair S,T, which we may shortly write (#, t ) ; and exactly the same formula represents the cylindric rope-drum, on which the rope is spirally coiled, the
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