The fly-wheel is a large wheel fixed on the crank shaft, and having a very heavy rim. As it revolves, this contains, stored up in itself, a great quantity of energy, and so equalizes the motion of the shaft, and, by restoring some of the energy, enables the engine to pass the " dead-points," or points at ivhich the connecting-rod and crank are in a line.
The condenser is simply a cast-iron box of any convenient shape. The water for con densing the steam is introduced into it in a jet in such a way that its particles mix with the steam at once on entering, and condense it almost instantaneously.
The governor is an ingenious application by Watt of mechanism long used in water. mills. Its object is to make the engine to a great extent regulate its own speed, so that it shall neither be pulled up altogether by a sudden increase of load, nor "race" when any part of its load is suddenly removed. It consists essentially of a spindle or upright rod, with a pully, by which it is caused to revolve, fixed on it. Two levers are pivoted on a pin near the top of the spindle, and at the lower end of each is fixed a heavy cast iron bell. When the engine is running at its proper speed, the balls revolve with the spindle in the position shown; but if that speed be increased, the centrifugal force causes them to fly outward, and consequently upward; and conversely, if it be decreased, they fall downward toward the center. At the upper end of the spindle is a system of levers, by which the raising of the balls tends to close, and their lowering to open, the throttle-valve. This valve is simply a disk of metal placed in the steam-pipe near the cyl inder. The further, therefore, it is opened, the greater the amount of steam admitted to the cylinder, and rice versa, and so the tendency of the engine to alter its speed arising from causes extraneous to itself, is jus.t balanced by the alteration made in the amount of steam admitted through the throttle-valve. In order that economy as well as regu larity of working may be attained, the governor should, however, be so arranged as to control the " cur-off" instead of throttling the steam.
The "Cornish" engine, so called from the fact that it is principally used in the Cor nish mines, resembles Watt's engine in general appearance. Like Newcomen's engine,
it is used exclusively for pumping, and general no rotary motion, and it is virtually singleacting; but unlike his, the steam pressure, and not that of the atmosphere, actually does the work. It is not easy to say why Cornish engines have remained so long in their original form. They are economical of fuel. owing to the great expansion used, but the same expansion could also be used with many other forms of engine. They are very costly, and extremely heavy and unwieldy, and it seems probable that it is only preju dice which stands in the way of their being superseded by small engines running at high speeds. which would do the same work as economically, and with a mtsch smaller outlay in first cost.
Engines in which the piston-rod and connecting-rod are directly attached arc called direcl-actiog engines, of which the horizontal engine is the most common type, and for all ordinary purposes is rapidly superseding every other form of stationary engine, It possesses the merits of having great simplicity and few working parts, and of all these parts being easily accessible to the engine-driver; and at the same time any required degree of economical working can be obtained in it as well as in any other form. It was for a long time only used as a non-coudensing (or so-called " high-pressure")engine, but is now generally made with a condenser attached.
Two other forms of direct-acting engines have been much used in their day, but are now being rapidly abandoned except under special circumstances; these are called respectively the "oscillating" and the "trunk" engine. In the former (which has rarely been used except for marine engines), the crank-shaft is above the cylinder, the piston-rod head, is attached to the cranl-pin, and the connecting-rod is dispensed with by allowing the cylinder to oscillate on large hollow centers called trunnions, and so to adapt itself to the various positions of the crank-pin. In the " trunk" engine, the piston rod becomes a hollow cylinder or trunk, large enough to allow the connecting-rod to vibrate inside it. The latter is then attached at one end to the crank-pin as usual, and at the other to a pin fixed in the piston.