Signedon back of scale: WATKINS & HILL / 5 CHARING CROSS / LONDON
FunctionThis is a rather rudimentary thermometer for measuring the intensity of solar radiation. When the bulb at the bottom is heated by solar radiation, the air inside pushes the liquid up the glass column, indicating the temperature of the bulb.
It cannot be very precise as the temperature of the bulb will also be affected by the air surrounding it.