Guide to the use of this subsection (classes ) The following notes are meant to assist in the use of this part of the classification scheme. In this subsection, subclasses or groups designating "engines" or "pumps" cover methods of operating the same, unless otherwise specifically provided for. In this subsection, the following terms or expressions are used with the meanings indicated: "engine" means a device for continuously converting fluid energy into mechanical power. Thus, this term includes, for example, steam piston engines or steam turbines, per se, or internal-combustion piston engines, but it excludes single-stroke devices. "Engine" also includes the fluid-motive portion of a meter unless such portion is particularly adapted for use in a meter; "pump" means a device for continuously raising, forcing, compressing, or exhausting fluid by mechanical or other means. Thus, this term includes fans or blowers; "machine" means a device which could equally be an engine and a pump, and not a device which is restricted to an engine or one which is restricted to a pump; "positive displacement" means the way the energy of a working fluid is transformed into mechanical energy, in which variations of volume created by the working fluid in a working chamber produce equivalent displacements of the mechanical member transmitting the energy, the dynamic effect of the fluid being of minor importance, and vice versa; "non-positive displacement" means the way the energy of a working fluid is transformed into mechanical energy, by transformation of the energy of the working fluid into kinetic energy, and vice versa; "oscillating-piston machine" means a positive-displacement machine in which a fluid-engaging work-transmitting member oscillates. This definition applies also to engines and pumps; "rotary-piston machine" means a positive-displacement machine in which a fluid-engaging work-transmitting member rotates about a fixed axis or about an axis moving along a circular or similar orbit. This definition applies also to engines and pumps; "rotary piston" means the work-transmitting member of a rotary-piston machine and may be of any suitable form, e.g., like a toothed gear; "cooperating members" means the "oscillating piston" or "rotary piston" and another member, e.g., the working-chamber wall, which assists in the driving or pumping action; "movement of the co-operating members" is to be interpreted as relative, so that one of the "co-operating members" may be stationary, even though reference may be made to its rotational axis, or both may move; "teeth or tooth equivalents" include lobes, projections or abutments; "internal-axis type" means that the rotational axes of the inner and outer co-operating members remain at all times within the outer member, e.g., in a similar manner to that of a pinion meshing with the internal teeth of a ring gear; "free piston" means a piston of which the length of stroke is not defined by any member driven thereby; "cylinders" means positive-displacement working chambers in general. Thus, this term is not restricted to cylinders of circular cross-section; "main shaft" means the shaft which converts reciprocating piston motion into rotary motion or vice versa; "plant" means an engine together with such additional apparatus as is necessary to run the engine. For example, a steam engine plant includes a steam engine and means for generating the steam; "working fluid" means the driven fluid in a pump and the driving fluid in an engine. The working fluid may be in a gaseous state, i.e., compressible, or liquid. In the former case coexistence of two states is possible; "steam" includes condensable vapours in general, and "special vapour" is used when steam is excluded; "reaction type" as applied to non-positive-displacement machines or engines means machines or engines in which pressure/velocity transformation takes place wholly or partly in the rotor. Machines or engines with no, or only slight, pressure/velocity transformation in the rotor are called "impulse type". In this subsection: cyclically operating valves, lubricating, gas-flow silencers or exhaust apparatus, or cooling are classified in subclasses , , , irrespective of their stated application, unless their classifying features are peculiar to their application, in which case they are classified only in the relevant subclass of classes ; lubricating, gas-flow silencers or exhaust apparatus, or cooling of machines or engines are classified in subclasses , , except for those peculiar to steam engines which are classified in subclass . For use of this subsection with a good understanding, it is essential to remember, so far as subclasses , , , , and , , , which form its skeleton, are concerned: the principle which resides in their elaboration, the classifying characteristics which they call for, and their complementarity. Principle This concerns essentially the subclasses listed above. Other subclasses, notably those of class , which cover better-defined matter, are not considered here. Each subclass covers fundamentally a genus of apparatus (engine or pump) and by extension covers equally "machines" of the same kind. Two different subjects, one having a more general character than the other, are thus covered by the same subclass. Subclasses , , , beyond the two subjects which they cover, have further a character of generality in relation to other subclasses concerning the different species of apparatus in the genus concerned. This generality applies as well for the two subjects dealt with, without these always being in relation to the same subclasses. Thus, subclass , in its part dealing with "machines", should be considered as being the general class relating to subclasses , , and in its part dealing with "engines" as being general in relation to subclass . Characteristics The principal classifying characteristic of the subclass is that of genera of apparatus, of which there are three possible: Machines; engines; pumps. As stated above, "machines" are always associated with one of the other two genera. These main genera are subdivided according to the general principles of operation of the apparatus: Positive displacement; non-positive displacement. The positive displacement apparatus are further subdivided according to the ways of putting into effect the principle of operation, that is, to the kind of apparatus: Simple reciprocating piston; rotary or oscillating piston; other kind. Another classifying characteristic is that of the working fluid, in respect of which three kinds of apparatus are possible, namely: Liquid and elastic fluid; elastic fluid; liquid. Complementarity This resides in association of pairs of the subclasses listed above, according to the characteristics under consideration in respect of kind of apparatus or working fluid. The subclasses concerned with the various principles, characteristics and complementarity are shown in the subsection index below. It is seen from this index that: For the same kind of apparatus in a given genus, the characteristics of "working fluid" associates: and to Machines and to Machines and to Machines and to Engines and to Engines and to Engines For the same kind of working fluid, the "apparatus" characteristic relates subclasses in the same way as considerations of relative generality. MACHINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only ENGINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only PUMPS positive displacement rotary or oscillating piston reciprocating piston or other non-positive displacement This subclass covers: combustion product or hot gas turbine plants; internal combustion turbines or turbine plants; turbine plants in which the working fluid is an unheated, pressurised gas. This subclass does not cover: steam turbine plants, which are covered by subclass ; special vapour plants, which are covered by subclass . In this subclass, the following expression is used with the meaning indicated: "gas-turbine plants" covers all the subject matter of Note (1) above and covers also features of jet-propulsion plants common to gas-turbine plants. Attention is drawn to the Notes preceding class .