What is "frost valve"

Frost \Frost\ (fr[o^]st; 115), n. [OE. frost, forst, AS. forst, frost. fr. fre['o]san to freeze; akin to D. varst, G., OHG., Icel., Dan., & Sw. frost. [root]18. See Freeze, v. i.]

1. The act of freezing; -- applied chiefly to the congelation of water; congelation of fluids.

2. The state or temperature of the air which occasions congelation, or the freezing of water; severe cold or freezing weather.

   The third bay comes a frost, a killing frost.
   --Shak.

3. Frozen dew; -- called also hoarfrost or white frost.

   He scattereth the hoarfrost like ashes.
   --Ps. cxlvii. 16.

4. Coldness or insensibility; severity or rigidity of character. [R.]

   It was of those moments of intense feeling when the frost of the Scottish people melts like a snow wreath.
   --Sir W. Scott.

   Black frost, cold so intense as to freeze vegetation and cause it to turn black, without the formation of hoarfrost.

   Frost bearer (Physics), a philosophical instrument illustrating the freezing of water in a vacuum; a cryophorus.

   Frost grape (Bot.), an American grape, with very small, acid berries.

   Frost lamp, a lamp placed below the oil tube of an Argand lamp to keep the oil limpid on cold nights; -- used especially in lighthouses.
   --Knight.

   Frost nail, a nail with a sharp head driven into a horse's shoe to keep him from slipping.

   Frost smoke, an appearance resembling smoke, caused by congelation of vapor in the atmosphere in time of severe cold.

   The brig and the ice round her are covered by a strange black obscurity: it is the frost smoke of arctic winters.
   --Kane.

   Frost valve, a valve to drain the portion of a pipe, hydrant, pump, etc., where water would be liable to freeze.

   Jack Frost, a popular personification of frost.