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The Steam Turbine 199 the steam at either end causing the piston to move to and fro rhythmically. The piston itself has an oscillating or reciprocating motion, but this becomes converted into rotary motion through the intervention of the crankshaft. The details of construction may undergo certain modifica- tion according to requirements. For instance, in the locomotive there is no flywheel in the generally accepted sense of the term, inasmuch as the driving- wheels are mounted upon the crankshaft. Yet the driving-wheels are really flywheels, and impel the locomotive forward by friction with the rail. Such constitutes the broad principles of the steam-engine’s operations, which from the to-and-fro motion of the piston has led to its being generically described as the reciprocating engine. In the rotary engine the idea is to apply the steam in such a manner as to give a direct rotary motion. There are no cranks, pistons, or piston-rods. What would be the crankshaft in the reciprocating engine becomes a straight driving-shaft in this instance. Rows of vanes or blades are mounted in rings upon it, and the steam playing upon these projections causes the blades to rotate. As these blades are fixed to the shaft they drag the latter round with them. At first sight it appears to be a simple, straightforward undertaking to cause a shaft to revolve in this manner ; but, as a matter of fact, the question is attended with innumerable difficulties of a highly technical and scientific character. These problems, coupled with a lack of essential knowledge concerning the scientific side of the subject, militated against the early experimenters.