It is a valve or opening through which steam may escape.
What prevents all of the steam from escaping through the valve?
The valve is kept closed by a weight, which prevents the escape of the steam until a certain quantity of it is formed in the boiler; it then lifts up the valve, and a portion of it escapes, the same as it does with the lid of the tea-kettle.
Evaporation.
Does a wet blanket, when hung in the air, remain wet?
No; it soon becomes dry.
What becomes of the water in the blanket?
The water passes off into the air.
What is this process called?
It is called Evaporation.