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The Collaborative International Dictionary
Snifting valve

Snifting \Snift"ing\, a. & n. from Snift.

Snifting valve, a small valve opening into the atmosphere from the cylinder or condenser of a steam engine, to allow the escape of air when the piston makes a stroke; -- so called from the noise made by its action.

Wiktionary
snifting valve

n. A small valve opening into the atmosphere from the cylinder or condenser of a steam engine, to allow the escape of air when the piston makes a stroke.

Wikipedia
Snifting valve

A snifting valve (sometimes snifter valve) is an automatic anti-vacuum valve used in a steam locomotive when coasting. The word Snift imitates the sound made by the valve.