All companies or corporations having or owning, or which may hereafter have or own any bridge or other structure spanning the Monongahela river, between its confluence with the Allegheny river and the borough of Elizabethtown, in the county of Fayette, are hereby required to keep a light or lights burning from dark till daylight, throughout the night, on each pier of each bridge or structure, so that said light or lights may be readily seen at all hours of the night by persons navigating said river.