What am I?

This dinky little tower seemingly faced in copper caught my eye on a recent trip to London. It's just down some steps between Westminster Bridge and Westminster Pier. It is apparently the ‘Thames Tide Recorder’ which to measures the depth of, and the tidal flow of the River Thames at one of the busiest sections of the river. The tidal range of the river is c. 6m (20 ft or so) at this point with the tidal stretch of the river ending at Teddington Lock. Find out more about the river in my history talk "Father Thames".