544 KINEMATICS OF MACHINERY.
fore an order for which we may use the symbol (T t J and one of which very frequent use is made in machine practice.
We have, lastly, pairings between two pressure organs, analogous to the pairing of T with T which we have already examined. These occur somewhat frequently in a form which we may express by the symbol (Q^,\), and of which we have illustrations in the air vessels of pumps and various hydraulic machines, in spiral pumps, hydraulic blowers and so on. We may indicate this order of pairs generally by the symbol
Recapitulation of the Pairs containing Flectional' Elements.
In summarising the pairs of elements considered in the last two sections it will be sufficient to tabulate the symbols for the orders alone, those for the classes and groups can be formed from these as in the case of the pairs of rigid elements. We have to add to the seven orders already tabulated :
(a) The six following orders containing tension-organs :
VIII. &) XL (tf, t ;)
IX. (HJ XII. (ff A :)
X. (tf.,0 XIII. (T t )
(b) The eight following orders containing pressure-organs :
XIV. (SO XVIII. (ft,:)
XV. (#0 XIX. (F,0
XVI. (F.O XX. (TJ
xvii. (ft z5 ) XXL (ft)
If it be required to indicate that the pressure-organ is a fluid, the symbol X or 7 (as the case may be) can be used instead of the q.
Examples of the majority of these twenty-one orders of pairs already exist in machinery, others of them have not yet found practical application. Our object here does not allow us to treat them further in detail. Our investigation has, however, gone far