514 KINEMATICS OF MACHINERY.
and employed to control the passage of the driver or the work- piece through the machine, to regulate, that is, the quantity of the material of either of them employed per unit of time according to the requirements of each instant. We may call this the regulating-gear or regulator. While the director determines the sequence of the motions in the machine, the regulator determines their quantity. The various governors of prime-movers of course belong to this class of mechanisms; they regulate the motion or supply of the driving-organ and consequently the speed of the whole machine. In the Cornish engine the cataract is the regulator, in the working-gear of clocks escapements of various kinds fulfil the same function. There is also regulating-gear in very many direct-actors, as in the regulators of looms and paper-making machines. Escape-valves placed upon air, steam or gas-pipes also regulate the supply of the fluid by preventing its pressure ever increasing beyond a certain fixed amount.
It is sometimes required, most often in direct-actors, that the regulator should be able entirely to stop the action of the driver ; as, for instance, when there is danger of any great irregularity occurring in the work produced by the machine. Begulating-gear acting in this particular way we may call stop-gear; it is made in very many forms. As illustrations of it we may mention ; the arrangements in the loom, which bring the machine to a stand if the weft thread does not pass ; those which stop it if any one of its numerous threads break ; the arrangements for shutting off the water from a hydraulic lift when the right height has been reached, and so on.
The regulator and the director are often very closely con- nected; in modern steam-engines, for example, the former (the governor) acts directly upon the latter (the valve-gear), and by means of it controls the motion of the driver (steam) ; it is always possible, however, to consider the two separately. When both of them or other sub-mechanisms exist special parts frequently (although not always) become necessary simply for the purpose of transmitting motion. This we may call the transmitting- gear, or more shortly the gearing of the machine. Gearing is also frequently inserted between a prime-mover and the direct- actors which it drives.
