The central arbor carrying the large blued-steel hand rotates on two wheels to reduce friction caused by the weight of the heavy hand. There is a massive Toricelli tube in the mahogany case with a float on the lower level, which is connected to a pulley on the central arbor balanced by a counterweight on the other side of the pulley. In this way the position of the hand is controlled by the changing level in the tube, which in its turn is dependent on the atmospheric pressure.
The hand indicates the air pressure on a large silvered and engraved brass dial. There is a setting hand on the outer rim, mounted on a large toothed wheel (408 teeth). The hand can be set by turning the gilt brass button to coincide with the barometer hand. When the air pressure changes, the pressure can be seen rising or falling in relation to the setting hand, which is an indication of the weather to come.
It is important with these instruments that the friction in the mechanism is minimal. This means that all turning parts are highly polished and not oiled as this make the mechanism sticky. And still the owner has to tap the glass to get the right reading.