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.

In “Die Naturwissenschaften,” 6th February, 1925, p. 93, there appears an article by Prandtl entitled “ Magnuseffekt und Windkraftschiff,” giving the following explanation of the phenomena under consideration.

In a stream of viscous fluid a non-rotating cylinder builds up an accumulation of retarded fluid with considerable vorticity (Fig. 5). This fluid reaches the neighbourhood of the near dead water point with a loss of kinetic energy and velocity by dissipation. At the somewhat indeterminate boundary of the boundary layer it is in contact with fluid which has lost its kinetic energy and velocity in acquiring an increase of pressure approaching the dead water pressure. The retarded fluid is pushed away from the dead water point by the higher pressure fluid (Figs. 6, 7), and ultimately it breaks up into cylindrical masses which are detached and carried away by the stream (Fig. 8).