noun: the part of a machine or piece of equipment that opens and closes in order to control the flow of air or liquid

Keely
"The maximum test was made to placing an iron weight of 580 lbs. on the extreme end of the long arm of the lever. To lift this weight required a pressure of 18,900 lbs. to the square inch counting the difference in the length of the two arms and the area of the piston. When Keely turned the valve-wheel leading from the receiver to the flexible tube and through it into the steel cylinder beneath the piston, simultaneously with the motion of his hand the weighted lever shot up against its stop a distance of several inches, as if the iron were cork. [Snell Manuscript - The Book, page 3]