When any person is about to descend or ascend a shaft or slope, the headman or footman, as the case may be, shall inform the engineer by signal or otherwise of the fact, and the engineer shall return a signal before moving or starting the engine. In the absence of a headman or footman the person or persons about to descend or ascend shall give and receive the signals in the same manner. At all shaft and slope landings where persons, coal and other material are hoisted or lowered by machinery a system of signaling or code of signals, approved by the mine inspector of the district, shall be posted.