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Relation of Pressure and Temperature at the Ben Nevis Observatories.* 

 By Alexander Buchan, LL.D, F.R.S. 



The object of this investigation was to ascertain the differences of atmospheric pressure,, 

 reduced to sea-level, which accompany differences of temperature, as observed at the 

 High and the Low-Level Ben Nevis Observatories. The period of time dealt with is 

 the ten months from August 1890 to May 1891. The whole of the hourly observa- 

 tions made at each Observatory with the barometer were reduced to sea-level, those at 

 the top of the mountain being reduced according to the table accompanying the Report 

 on the Meteorology of Ben Nevis, t 



It has been shown that the influence of high wind on the barometer is to lower the 

 readings, the amount of the depression increasing as the velocity of the wind increases. 

 It is, however, only when the wind exceeds force 3 on Beaufort scale, to 12, or 

 21 miles an hour, that the barometer is lowered more than the one-hundredth of an 

 inch. Hence in this discussion the higher velocities of the wind were wholly left out, 

 and only such pressures and temperatures were dealt with as occurred when either calms 

 or light winds prevailed at the time. 



During the ten months examined, the differences of temperature between the two 

 Observatories ranged from a temperature 26°"0 lower, to a temperature 6 o, higher, 

 than was recorded at Fort- William at the same hour. A comparison was then made 

 by sorting the differences into two-degree amounts, thus making sixteen columns in 

 all, the first column including those observations when the temperature at the top was 

 from 4° - to 6°"0 higher than at Fort- William ; the second column when from 2°'0 to 

 4 o- higher; and so on to the sixteenth column, which included the observations when 

 the temperature of the top was from 24 o- to 26 o, lower than at Fort- William. In 

 these columns the differences, plus or minus, between the two barometers, after being 

 reduced to sea-level, were entered. 



The results are summarized in the following table, which shows for each two-degrees 

 difference of temperature, the difference between the reduced barometer of the top and 

 the barometer at Fort- William, the plus sign indicating that the top temperature, or 

 top barometer, was the higher of the two, and the minus sign that it was the lower. 



There were 5037 comparisons made. On 1645 of these cases, the reduced barometer 

 of the top was the higher of the two, and on 3392 cases it was the lower. On fifty-five 

 cases the temperature difference varied from the upper thermometer, being from 6 o, 

 higher to only 2 o, lower than at Fort- William ; and on every one of the cases the 

 reduced barometer of the top was the higher of the two. On the other hand, there 

 were 254 cases where the upper temperature was from 22° "0 to 26° *0 lower than 



* See Journal of Scottish Meteorological Society, vol. ix. p. 137. 

 t Trans. Roy. Soc. Edin., vol. xxxiv. pp. Ix., lxi. 



