The extremely ingenious modern mechanism of the Newgate Street Clock was devised by Smith of Derby. Unlike the 17th century  wandering hour clocks and watches it imitates, it is kept in motion by electro-mechanical means and corrected by satellite.

The first illustration shows a central rotating arm, designed to turn once in two hours

The second illustration shows two hour disks, numbered with alternate hours, which are carried round by the central arm.

The third illustration shows a large blue disc, also attached to the central rotating arm, which allows only two of the hours to be seen at any one  time.

The fourth illustration shows the polished steel outer dial. This remains static and allows only the upper rotating number to be  visible to the passer by.

Thus the hour rises in the  upper aperture of the fixed steel dial and travels slowly round in a clockwise direction, pointing to the minutes as it goes. Before it sets, the second hour disc, then out of sight, turns one sixth to show the next correct hour. As one hour sets, the next rises.