instead of one with a nut (Fig. 44 J), it is simply an inversion of the closed pair screw and nut. In the common waggon wheel the axle is fixed to the waggon body, the wheel with the open revolute moving upon it; in a railway carriage the latter is attached to the frame, the solid body or shaft being con- nected with the wheel, and movable. For a guide for rectilinear motions either of the arrangements shown in Figs. 45 and 46 is used, as may be more convenient; in the first of these a solid prism A slides in a prismatic slot, or open prism, B, while in the other an open prism, A, is moved backwards and forwards upon a straight bar B.
The familiar and easy realization of this invertibility of the elements of closed pairs is in many cases of the greatest value
Fig. 47. FIG. 48.
to the constructor; in using such pairs he at once recognises the possibility of employing either the one or the other element as the hollow body, or making the contact of each with its partner partly internal and partly external. The recognition of this principle sometimes removes differences between constructions which in their external appearance differ more or less widely, or at least gives a simple expression to what was before an indistinct sense of relationship between them. There may be mentioned, for instance, the exchange, which has of late been frequently seen, of the cylinder with the piston, which (e.g.) distinguishes Condie's Steam-hammer from Naisinyth's. What the constructor has here carried out is the inversion of a prism-pair, whilst in other matters the functions of the different mechanisms remain
