An attempt is made in this chapter to chronicle the circumstances under which the successive developments of the steam engine took place. The progress of the scientific ideas which led up to the discovery of the power of steam is traced. The claims of the various inventors chiefly associated with the steam engine are set forth in some detail, not for the difficult and invidious task of assessing their relative merits, but because by the light of these claims and altercations it may be possible to discern, in each case, where the merit lay and to what stage each novelty of idea or detail properly belonged. From this point of view, it is thought, the recital of circumstances which hitherto have been thought so trivial as to be scarcely worthy of record, may be of some suggestive value. The result of the investigation is to make clear the scientific importance of the steam engine: the steam engine regarded, not as the familiar drudge and commonplace servant of to-day, but in all its dignity of a thermodynamic machine, that scientific device which embodied so much of the natural philosophy of the age which first unveiled it--the seventeenth century. § Before the Christian era steam had been used to do mechanical work. In a treatise, _Pneumatica_, written by Hero of Alexandria about 130 B.C., mention is made of a primitive reaction turbine, which functioned by the reactionary force of steam jets thrown off tangentially from the periphery of a wheel. In the same work another form of heat-engine is described: an apparatus in which, by the expansion from heating of air contained in a spherical vessel, water was expelled from the same vessel to a bucket, where by its weight it gave motion mysteriously to the doors of temples. And evidence exists that in these two forms heat engines were used in later centuries for such trivial purposes as the blowing of organs and the turning of spits. But except in these two primitive forms no progress is recorded for seventeen centuries after the date of Hero’s book. The story of the evolution of steam as a motive force really begins, with the story of modern science itself, at the end of the Middle Ages.