The progress of mechanical flight to the present stable position occupied by aircraft as a means of transport has been so closely related with the problems associated with the development of the power unit, that an examination of such an important factor as its “ reliability ” appears justified.In searching for possible means to produce power, it was natural that the use of steam as a motive power should have been considered. First Langley and then Maxim in 1895 produced a steam plant designed for aircraft. No practical success was attained by these early efforts, and a considerable period elapsed before the successful power unit for aircraft was evolved.