English: Two antique precision regulator clocks at the Astronomical Observatory of the University of Rio Grande do Sul. The center clock, manufactured by Adolf Opperman. Berlin, Germany, has a mercury pendulum which compensates for temperature induced changes in the length of the pendulum rod. Astronomical regulator clocks minimized error due to gear friction by having the simplest possible gear train. To simplify it, rather than have the hands mounted on concentric shafts they are mounted on the shafts of separate gearwheels, so the face has three separate dials for seconds, minutes and hours. The largest dial is for the second hand.