This turbine engine constitutes the first really important departure from the old-type steam engine, thus realizing the dream of the seventeenth-century Italian, Branca, to which reference was made above. Mr. Parsons' elaboration of the idea developed a good deal of complexity as regards the number of parts involved, yet his engine is of the utmost simplicity in principle. It consists of a large number of series of small blades, each series arranged about a drum which revolves. Between the rings of revolving blades are adjusted corresponding rings of fixed blades, which project from the casing to the cylinder, and by means of which the steam is regulated in direction, so that it strikes at the proper angle against the revolving blades of the turbine. In practise, three series of cylindrical drums are used, each containing a large number of rings of blades of uniform size; but each successive drum having longer blades, to accommodate the greater volume of the expanding steam. The steam is fed against the first series of blades in gusts, which may be varied in frequency and length to meet the requirements of speed. After impinging on the first circle of blades, the steam passes to the next under slightly reduced pressure, and the pressure is thus successively stepped down from one set of blades to another until it is ultimately reduced from say two hundred pounds to the square inch, to one pound to the square inch before it passes to the condenser and ceases to act. There is thus a fuller utilization of the kinetic energy of the gas, through carrying it from high to low pressure, than is possible with the old type of cylinder-and-piston engine. On the other hand, there is a constant loss due to the fact that the blades of the turbine can not fit with absolute tightness against the cylinder walls. The net result is that the compound turbine, as at present developed, appears to have about the same efficiency as the best engine of the old type.