In the reciprocating type of steam engine the inertia of the piston must be overcome at the beginning of each stroke and its momentum must be arrested at the end of each stroke, and this involves a great loss of power. If the power of the steam could be applied so as to continuously move the piston in the same direction this loss would be avoided. The effort to do this has engaged the attention of many inventors, and the devices are called rotary engines. The most successful engines of this kind are those of the impact type, in which jets of steam impinge upon buckets after the manner of water on a water wheel, and which are known to-day as steam turbines. The earliest of these is Branca’s steam turbine of 1629 (see Fig. 85) and the most important of this class in use to-day are those of Mr. Parsons, of England, and De Laval, of Sweden. The internal construction of the Parsons turbine is seen in Fig. 86 and is covered by British patent No. 10,940, of 1891, and United States patent No. 553,658, January 28th, 1896. A series of turbines are set one after the other on the same axis, so that each takes steam from the preceding one, and passes it on to the next. Each consists of a ring of fixed steam guides on the casing, and a ring of moving blades on the shaft. The steam passes through the first set of guides, then through the first set of moving blades, then through the second set of guides, and then through the second set of moving blades, and so on. [Illustration: FIG. 87.--PARSONS COMPOUND STEAM TURBINE, ON PLURALITY OF PROPELLER SHAFTS.] In the application of his turbine to marine propulsion Mr. Parsons employs a plurality of propeller shafts and steam turbines, as seen in Fig. 87, and covered under United States patent No. 608,969, August 9, 1898. [Illustration: FIG. 88.--DE LAVAL’S STEAM TURBINE.] [Illustration: FIG. 89.--DE LAVAL TURBINE GEARED TO DYNAMO.]