All bridge tenders and other persons in charge of drawbridges in the City shall always be on duty and constant watch during the hours assigned to them, and shall, as soon as they hear the signal given by any tug or other steam craft for their bridge to be opened, immediately run up the red ball during the day, or a red lantern during the night, and in addition to the abovementioned signals ring the bell, or give such other signal as may be on the bridge, so that the persons in charge of the approaching craft may know that the bridge is to be opened for them to pass through. The red ball or light shall be lowered as soon as the vessel has passed the draw. Whoever violates this section shall be subject to dismissal.