Thanks to the people on my email network who identified the equipment in the "odd convection experiment" as Hope's Apparatus. As I understand it, if I kept shovelling ice into the middle bit the bottom thermometer would reach 4 degrees Celsius and no lower whilst the water remained liquid. This is because water is most dense at that temperature and the most dense will sink to the bottom. And Thomas Hope was the scientist who discovered the element Strontium. Some years ago I visited the village of Strontian after which it is named.