VALVE, in Hydraulics, Pneumatics, &c. is a kind of lid or cover of a tube or veſſel, ſo contrived as to open one way, but which, the more forcibly it is preſſed the other way, the cloſer it ſhuts the aperture; ſo that it either admits the entrance of a fluid into the tube or veſſel, and prevents its return; or admits its eſcape, and prevents its re-entrance.