The form of the turbine was that of a rotating drum, with outwardly projecting rings of blades which nearly touched the containing cylindrical case, and on the case inwardly projecting rings of guide blades which nearly touched the drum. In the first examples of the engine there were two groups of turbines right-and left-handed on each side of the steam inlet, the exhaust taking place at each end of the turbine case, so as to completely balance end pressure from the steam. More recently one series of turbines only has been used, those on the other side of the steam inlet being replaced by packing rings or rotating balance pistons which balance the end pressure and divert the whole of the steam through the turbines on the other side. [Illustration: FIG. 2 SHOWS THE ARRANGEMENT OF MOVING BLADES AND GUIDE VANES IN A PARSONS’S TURBINE. The top outer cover has been removed. The cylinder containing the revolving barrel has, as will be seen, a greater internal diameter than the diameter of the drum. It is the annular space thus formed through which the steam flows and which contains the revolving blades and the fixed guide blades. Between each two rings of moving blades there is a ring of guide blades, the latter being keyed into the containing case. The vanes are set at an angle, so that the steam acts on them as wind on the sails of a windmill.] The steam entering the annular space between the shaft and the case passes firstly through a ring of guide blades attached to the case, and is given a rotational direction of flow; it then passes to the succeeding ring of blades attached to the shaft, by which its direction of rotation is reversed, thereby impressing the difference of its rotational momentum in torque to the shaft. The steam then passes to the second ring of guide blades, and the process is repeated, and so on, gradually expanding by small increments at each ring of blades; the succeeding rings of blades get longer and wider, and at intervals the diameter of the turbine drums, cylinders, and rings are also increased. In condensing turbine engines of the larger size an expansion ratio in the turbines of one hundredfold and upward is attained before the steam passes to the exhaust pipe and condenser.