Evaporation hook gauge and still well for measuring evaporation rates, numbered W5822 and W5824 respectively, by Casella, London, UK, 1986.

This device is for measuring the rate of evaporation. The hook component is placed - hook pointing downward - into a well of water and the length of the shaft adjusted using the dial at the top of the instrument, so that the upwards-facing tip of the hook just penetrates the water surface. An initial reading is taken from the instrument's gauge. After a given period, by which time some of the water in the well would have evaporated, the process is repeated, adjusting the dial to reposition the hook.