The original air engines (also known as a heat, hot air, caloric, or Stirling engines), predated the modern internal combustion engine. However, the primary obstacle to its practical use in the past has been the lack of sufficiently heat-resistant materials, which has now been eliminated due to the higher strength of modern materials and alloys. Several companies in the U.S. and abroad are successfully marketing new machines based on the air engine concept. The historical portion of the book is based on four famous articles he published in 1959. The rest of the chapters assess the development of the air engine and put it in the modern context, as well as investigate its future potential and applications.