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 In this class, the following expressions are used with the meanings indicated: "heat exchange" means the heating or cooling of a fluid or fluent solid by direct or indirect contact with a heated or cooled fluid or fluent solid; "heat transfer" means the heating or cooling of a fluid or fluent solid by direct contact with a heated or cooled surface or body. Apparatus using heat exchange or heat transfer (as defined in Note (1) above) for specific purposes is classified either in subclass or in the appropriate subclasses of, for example, classes , , , , or ; if no such other subclass is appropriate, such apparatus is classified in subclass or .