For a few years readings of a Black Bulb in vacuo thermometer formed part of the regular hourly routine work at the Observatory on Ben Nevis during the summer months, and a summary of two years' observations, for 1890 and 1891, is given in this paper. The instrument consists of the usual vacuum chamber, a spherical glass globe about 2½ inches diameter, with inside, centrically, a thermometer whose bulb and an inch or so of stem are coated a dull black. The thermometer is an ordinary, not a maximum Fahrenheit thermometer, and therefore gives the Black Bulb temperature at the instant of reading only. It was exposed on a wooden stand in a horizontal position, with the bulb projecting beyond the frame, and pointing south, about 4 feet above the broken stones which form the hill top, and was read immediately after the shade temperature during daylight hours.