6. A nicely calculated and adjusted balance of reciprocating parts.
7. Security against injury by shock, whether due to the presence of water in the cylinder or to looseness of running parts.
8. A “positive-motion” cut-off gear.
9. A powerful but sensitive and accurately-working governor determining the degree of expansion.[118]
10. Well-balanced valves and an easy-working valve-gear.
11. Small volume of “dead-space,” or “clearance,” and properly adjusted “compression.”
It would seem sufficiently evident that the engine with detachable (“drop”) cut-off valve-gear must, sooner or later, become an obsolete type, although the substitution of springs or of steam-pressure for gravity in the closing of the detached valve may defer greatly this apparently inevitable change. The “engine of the future” will not probably be a “drop cut-off engine.”
As regards the construction of the engine as a piece of mechanism, the principles and practice of good engineering are precisely the same, whether applied in the designing of the compound or of the ordinary type of steam-engine. The proportioning of the two machines to each other in such manner as to form an effective whole, by procuring approximately equal amounts of work from both, is the only essential peculiarity of compound-engine design which calls for especial care, and the method of securing success in practice may be stated to be, for both forms of engines, as follows:
1. A good design, by which is meant—
a. Correct proportions, both in general dimensions and in arrangement of parts, and proper forms and sizes of details to withstand safely the forces which may be expected to come upon them.