Ancient Roman steelyard, from Pompeii, Italy, 79 AD.
© Science Museum / Science & Society Picture Library
Like other balances, this device measures mas rather than weight. Since the force of gravity is the same at both ends of the beam, which acts as a lever, the measurement that it gives would be the same on Earth or on the Moon. It does not depend on gravity. The head-shaped weight is moved along the beam until it balances with the object whose mas is being measured. The mas is shown by a scale along the beam.