Whenever the steam-engine is employed to execute any work which is variable in its quantity or intensity, there must be some means of adjusting the force of the engine to this varying resistance it has to overcome : if the engine were suddenly relieved of half that resistance, as, for example, would be the case if half the machinery it was driving were suddenly stopped or thrown out of gear, the engine, if impelled by the same quantity of steam, would immediately move with a rapidly accelerating velocity, till that velocity were doubled, which would be prejudicial to the engine itself and to the rest of the machinery it was working, as well as be a useless expenditure of forco or fuel. The object of that beautiful piece of mechanism termed the governor, is to enable the engine to regulate the supply of steam admitted to the The governor, the form and principle of which will be better under stood from the figure (fig. 14) than from any description, is made to revolve on its vertical axis a, by a band passing round a pulley and round one on the axle of the fly-wheel, or else by bevilled wheels, as shown in the figure ; so that its velocity of rotation varies with that of the fly-wheel. If this velocity increase, the heavy balls b b diverge by the increased centrifugal force, and cause the collar c to slide up the axis ; this by means of the intermediate cranks partially closes the valve at v, in the steam-pipe a, termed the throttle-valve, and consequently diminishes the quantity of steam passing to the cylinder : if, on the contrary, the velocity of the fly-wheel is diminished by an increase in the resist ance, the balls of tho governor collapse, and the throttle-valve is opened so as to admit more steam to the cylinder to augment the force in proportion to the increased resistance.
The effect on the governor is equivalent to varying the diameter of a second fly-wheel, the circumference of which is represented by the balls b b, consequently the governor itself, independently of its action on the throttle-valve, has a tendency to effect the adjustment required ; since a portion of the surplus force of the engine, on the first supposi tion, is absorbed in overcoming the increased inertia of the governor ; while, on the second supposition, that inertia, being diminished, relieves the engine of a portion of resistance. This accounts for the prompt action of the governor in effecting the adjustment, which it does with out those fluctuations in the velocity of the piston which would be prejudicial to the effect of the machinery actuated by this prime mover.
In the marine engine no governor is requisite; the resistance being so great compared to the force, that the velocity can never be exces sive, and all the power the engine is capable of exerting is required to turn the paddlewheels; added to which the resistance is nearly uni form, only varying with the draught of the vessel or the state of the weather, but never diminishing below that originally calculated on and provided for by the power of the engines ; an increase of the resistance accordingly produces a diminution in the velocity of the vessel, but the engine can never work so fast as to be injured by its own velocity.
The governor is equally unnecessary to the locomotive engine, since the attendant must constantly have the engine under his control, and be watching its action ; he can therefore regulate the throttle-valve by hand•gear placed within his reach ; and in this case also, as with the marine engine, the resistance is nearly constant, consisting chiefly in the weight of the engine and the train of carriages to be moved, a weight not liable to any sudden change during the transit, while the momentum of the whole mass acts as a fly-wheel or regulator to equalise the motion of the pistons.
We have not entered into any investigations of the formuke for determining the dimensions of an engine and of its boiler, the pressure at which it must be worked, the velocity of the piston, &c., in order to produce any proposed available or net force ; or into the account of the experiments by which the amount of friction, the effects of the uncondensed steam, the quantity of caloric obtained from different kinds of fuel, &c., all of which must be taken into account in these investigations, and have been approxirnatively determined.
(In the precedini article the formulas expressing tho physical and mechanical properties of steam will be found, and fur the others the following works among others may be consulted : Tredgold, T., On the .ztearn•Engine ; Farey, J., On the b'tearn-Enylne, Ito.; De Tambour, F.31.11, Chess, Theorie de to Machine a l'aprnr, Svo., Paris; Galloway, Os the Stearn-Engine, and others.)