Nearly a hundred of these curious artifacts have been found across Europe. So far all have dated from the mid- to late-Roman Empire, specifically the 2nd and 3rd centuries AD. What makes them so interesting is that no contemporary accounts or images exist to explain their purpose. Hypotheses range from simple candlestick holders to calibration devices to primitive, oil-filled grenades. One of the more elaborate theories posits their use as agricultural almanacs, to determine the right day to plant or harvest. See here for more info on that.