These bridges allow horses towing boats to cross to other side of the canal

The one shown above crosses the Macclesfield Canal in east Cheshire.

A roving bridge, changeline bridge, turnover bridge, or snake bridge is a bridge over a canal constructed to allow a horse towing a boat to cross the canal when the towpath changes sides. This often involved unhitching the tow line, but on some canals they were constructed so that there was no need to do this by placing the two ramps on the same side of the bridge, which turned the horse through 360 degrees. On the Macclesfield Canal this was achieved by building spiral ramps and on the Stratford-upon-Avon Canal and others by constructing roving bridges of iron in two cantilevered halves, leaving a slot in the middle for the tow rope. This was also called a split bridge. For cost reasons many ordinary Stratford bridges were also built in this way as they had no towpath.

The ramps of the bridge are typically studded with alternating rows of protruding bricks to prevent the feet of the horse from sliding. The bridge may be constructed of cast iron (particularly in industrial areas) or of more conventional brick or stone.

Here's another one:

Addendum:  An enjoyable (and relevant) video found by reader Gelvan Tullibole 3rd:

What a pleasant way to spend a summer's day.

Addendum:  Reader nb Amy Jo offers this explanatory video of how a crossover bridge works -

I was pleased to see that "Ian received no harm during the making of this film."