Pl. IV.
WATT'S SINGLE ACTING STEAM ENGINE.
Drawn by the Author.
The peculiar beauty of this apparatus is, that in whatever position the balls settle themselves, the velocity with which the governor revolves must be the same,[21] and in this, in fact, consists its whole efficacy as a regulator. Its regulating power is limited, and it is only small changes of velocity that it will correct. It is evident that such a velocity as, on the one hand, would cause the balls to fly to the extremity of their play, or, on the other, would cause them to fall down on their rests, would not be influenced by the governor.
We have thus described the principal parts of the double-acting steam engine. The valves and the methods of working them have been reserved for the next chapter, as they admit of considerable variety, and will be better treated of separately. We have also reserved the consideration of the boiler, which is far from being the least interesting part of the modern steam engine, for a future chapter.
CHAPTER VII.
DOUBLE-ACTING STEAM ENGINE
(continued.)
On the Valves of the Double-Acting Steam Engine. — Original Valves. — Spindle Valves. — Sliding Valve. — D Valve. — Four-Way Cock.
(60.) The various improvements described in the last chapter were secured to Watt by patent in the year 1782. The engine now acquired an enlarged sphere of action; for its dominion over manufactures was decided by the fly-wheel, crank, and governor. By means of these appendages, its motions were regulated with the most delicate precision; so that while it retained a power whose magnitude was almost unlimited, that power was under as exact regulation as the motion of a time-piece. There is no species of manufacture, therefore, to which this machine is not applicable, from the power which spins the finest thread, or produces the most delicate web, to that which is necessary to elevate the most enormous weights, or overcome the most unlimited resistances. Although it be true, that in later times the steam engine has received many improvements, some of which are very creditable to the invention and talents of their projectors, yet it is undeniable that all its great and leading perfections, all those qualities by which it has produced such wonderful effects on the resources of these countries, by the extension of manufactures and commerce,—those qualities by which its influence is felt and acknowledged in every part of the civilized globe, in increasing the happiness, in multiplying the enjoyments, and cheapening the pleasures of life,—that these qualities are due to the predominating powers of one man, and that man one who possessed neither the influence of wealth, rank, nor education, to give that first impetus which is so often necessary to carry into circulation the earlier productions of genius.
The method of working the valves of the double-acting steam engine, is a subject which has much exercised the ingenuity of engineers, and many elegant contrivances have been suggested, some of which we shall now proceed to describe. But even in this the invention of Watt has anticipated his successors; and the contrivances suggested by him are those which are now almost universally used.
In order perfectly to comprehend the action of the several systems of valves which we are about to describe, it will be necessary distinctly to remember the manner in which the steam is to be communicated to the cylinder, and withdrawn from it. When the piston is at the top of the cylinder, the steam below it is to be drawn off to the condenser, and the steam from the boiler is to be admitted above it. Again, when it has arrived at the bottom of the cylinder, the steam above is to be drawn off to the condenser, and the steam from the boiler is to be admitted below it.
In the earlier engines constructed by Watt, this was accomplished by four valves, which were opened and closed in pairs. Valve boxes were placed at the top and bottom of the cylinder, each of which communicated by tubes both with the steam-pipe from the boiler and the condenser. Each valve-box accordingly contained two valves, one to admit steam from the steam-pipe to the cylinder, and the other to allow that steam to pass into the condenser. Thus each valve-box contained a steam valve and an exhausting valve. The valves at the top of the cylinder are called the upper steam valve and the upper exhausting-valve, and those at the bottom, the lower steam valve and the lower exhausting-valve. In [fig. 15]. A´ is the upper steam valve, which, when open, admits steam above the piston; B´ is the upper exhausting-valve, which, when open, draws off the steam from the piston to the condenser. C´ is the lower steam valve, which admits steam below the piston; and D´, the lower exhausting-valve, which draws off the steam from below the piston to the condenser.