echanical Engineers are present in virtually every industry.  Most fundamentally, they apply the principles of two of the fields of physics--mechanics and heat--to the  design of machines.  Heat is the principal form in which we use energy, so mechanical engineering is fundamental to all processes in which energy is created and used.  The field of mechanics is divided into the subfields of solid mechanics and fluid mechanics, and both of these subfields are basic to mechanical engineering.  A few of the myriad activities in which mechanical engineers engage are engine design, whether for autos, jets, diesel locomotives, or lawn mowers; rocket propulsion; combustion research; rapid transit systems, such as the new subways under construction in some U.S. cities; earth-moving machinery; air-conditioning systems; wind energy and solar energy devices;  aerospace vehicles; turbines for electric power generation; automatic control for rolling mills; farm machinery; typewriters; computer input--output devices; prosthetic devices and artificial limbs; artificial hearts; precision measuring equipment; printing presses; food processing systems; and pumps, whether to circulate the water in a swimming pool or to drive the coolant through a nuclear power plant.  The list is almost endless, but wherever a machine is needed to create a motion, to move a load, to create energy, or to convert it, there you will find mechanical engineers at work.

One type of mechanical engineer is the air-conditioning and refrigeration engineer, who designs systems to produce controlled indoor environments.  Most often, the designers of these mechanical systems are members of consulting firms and must work closely with architects and structural engineers in developing building plan.  Closely related are the activities of the sales engineer, who works with designers to supply the best equipment for their needs, and the building systems engineer, who is responsible for the continued effective performance of the systems after they are installed.

(Excerpts from:  John Dusting Kemper, "Introduction to the Engineering Profession" Second Edition, p. 165)