+33615 This refers to the length of a column of mercury used to measure pressure (or pressure drop).
Pressure drop of a liquid can be expressed as Δp = ρgh where ρ is density, g is gravitational acceleration and h is the height of the column of liquid. Sometimes, just the value of h is given (though this is a rather dated practice these days) and is referred to as a "head" of pressure. With mercury, the head is usually given in centimetres or millimetres, but if water is the liquid it was often given in feet. Thus, for water at least, "heads" were measured in feet!
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+33615 This refers to the length of a column of mercury used to measure pressure (or pressure drop).
Pressure drop of a liquid can be expressed as Δp = ρgh where ρ is density, g is gravitational acceleration and h is the height of the column of liquid. Sometimes, just the value of h is given (though this is a rather dated practice these days) and is referred to as a "head" of pressure. With mercury, the head is usually given in centimetres or millimetres, but if water is the liquid it was often given in feet. Thus, for water at least, "heads" were measured in feet!
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