History and What It Became

The physical principle behind the hydrostatic balance, that a submerged object displaces its own volume of water and is buoyed up by the weight of that displacement, is credited to Archimedes in the third century BCE, but building an instrument sensitive enough to put that principle to reliable, repeatable use took much longer. Al-Khazini's Book of the Balance of Wisdom, completed in 1121, is the fullest early treatise on the subject, an encyclopedic work of eight books and fifty chapters covering the theory of statics and hydrostatics behind the hydrostatic balance and describing his own instrument, a five-pan balance spanning roughly two meters, alongside the instruments of his predecessors, including al-Biruni's pycnometer flask.

Al-Biruni and al-Khazini are recognized by historians of science as among the first to bring genuinely experimental precision to the study of specific weights, synthesizing earlier Greek and Islamic theory into a working, repeatable measurement method. That method needed no further reinvention to remain useful into the modern era: determining an object's specific gravity by weighing it in air and then in water is still exactly how density measurements are made in chemistry, mineralogy and materials science laboratories today, using more precise instruments but the identical two-weighing principle al-Khazini set out in the twelfth century.