This section is from the book "The Home Cyclopedia Of General Information", by Charles Morris. Also available from Amazon: Home Cyclopedia of Necessary Knowledge.
[L. valva, the leaf of a folding door.] A kind of flap or lid in a pipe or a blood-vessel which allows a fluid or gas to flow only in one direction, as in the common pump. There are a flap-valve, puppet-valve, ball-valve and slide-valve. A safety-valve is held shut by a spring or weight, and opens automatically to allow steam, gas, or water to escape when the pressure becomes too great. The valves in the blood-vessels assist the flow of blood through the veins.
 
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