They are rollers to protect the canal bridges.
It's from the days when barges where horse drawn, as the horse pulls and perhaps goes around a corner the ropes would gradually wear into the stone or brick work, so rollers were fitted to reduce wear and tear.
You can actually see in your first pic' the way the ropes have worn the stone.
The second one was for carrying pipes across the canal from when the first sewer pipes were added after all back when the canals where first made they couldn't have planned any room in the bridges for them sewers didn't exist.