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Hourly Readings of Black Bulb in vacuo at Ben Nevis Observatory.* 



By R. T. Omond. 



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 2-^ 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. Readings at night were found to be useless, as the outer glass 

 jacket acted as a practically opaque screen to the low temperature rays, by which the 

 bulb would have parted with its heat and been lowered below the shade temperature 

 at night. It is impossible to work the instrument in winter, except during brief 

 intervals of sunny weather, as the least fog or snow coats the outer bulb with ice 

 and prevents the radiation action taking place. What exactly the Black Bulb 

 indicates is an open question, and, indeed, it is rare to find two instruments recording 

 the same,— slight unavoidable differences in manufacture alter the scale, so to speak ; 

 but as one instrument only has been used throughout in these readings, it may at least 

 be assumed to show the times of maximum solar radiation, and little more than the 

 determination of these times is attempted in this paper. The hours of observation are 

 Greenwich time, and the Black Bulb is read at between two and three minutes after 

 each hour; the times of local noon for Ben Nevis are approximately, in June, 12.20, in 

 July and August, 12.25, and in September, 12.15 p.m. 



In the table is given the shade temperature, and the difference between the mean 

 Black Bulb and shade temperature, at each hour on the average of the four summer 

 mouths. The diurnal range in the former is very small, amounting in each month to 

 only between 3° and 4° ; while the difference between shade and sun temperatures 

 ranges from 0° to over 40°. In every month the highest value is at 11 a.m., and most 

 of the months show a tendency towards a secondary maximum in the afternoon, either 

 by an actual rise, as in July, at 2 p.m., or by a decrease in the rate of fall, a flattening 

 of the curve sometime in the afternoon. Thus the highest readings are got about an 

 hour and a half before local noon, and there is a slight tendency to rise again an hour 

 or two after that time. We cannot suppose that the suu is really more powerful at 



* See Journal of Scottish Meteorological Society, vol. ix. p. 256. 



