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PRESSURE
1 kilo per square centimetre = 14'228 lbs. per square inch.
1 lb. per square inch = *0703 kilos per square cm.
Steam rising from water at its boiling point (212") has a
pressure equal to the atmosphere (14"7 lbs. to the sq. inch).
To evaporate one cubic foot of water requires the consump
tion of 7 1/2 lbs. of ordinary coal, or about 1 lb. of coal to
1 gallon of water.
One-sixth of tensible strength of plate multiplied by thickness
of plate and divided by one-half the diameter of boiler gives
safe working pressure for tubular boilers. For marine boilers
add 20 per cent for drilled holes.
No plate or bars of either steel or iron should be worked at a
black or blue heat (say about 500"); the material will stand far
more strain either red hot or cold, while at an intermediate
point great risks will be run and possible strains produced which
result in rupture later on.
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