It has been found that the discharge of steam through an opeuing into a pressure less than three-fifths the initial is about 900 ft. per second. The following rule has been given to determine the velocity in feet per second when steam flows into a vacuum: - To the Fahrenheit temperature of the steam add 4G0, and multiply the square root of the sum by 60. The area of pipes for steam engines should be arranged so that the velocity of the steam does not exceed 130 ft. per second; a lower velocity is better.