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Differences betiveen the Ben Nevis and Fort- William Barometers when both are 

 reduced to Sea-Level. By R. T. Omond. 



The barometer at the Fort- William Observatory is 42 feet above sea-level, and its 

 readings are reduced to sea-level by the ordinary tables based on Laplace's formula ; 

 but in reducing the barometer at the Ben Nevis Observatory, which is 4407 feet above 

 sea-level, use is made of a table specially constructed by Dr Buchan, and described by 

 him in the first volume of the Ben Nevis Observations (see Transactions, vol. xxxiv. 

 p. 24). This table consists of the average values of the difference between the barometer 

 on Ben Nevis and that at sea-level in Fort- William, for each successive tenth of an 

 inch of sea-level pressure and each degree of air temperature. The table is thus purely 

 -an empirical one ; it is the smoothed averages of a series of observations, and is not 

 based on any preconceived theory, nor on data drawn from any other source. A 

 reprint of the table will be found at the end of the Appendix. The table closely agrees 

 with the recognised values for the expansion of air with rise of temperature, a result 

 which indicates that the air temperatures used in it, namely, the average of the Ben 

 Nevis and Fort- William temperatures, represent very closely the actual average 

 temperature of the whole mass of air above Fort- William up to the level of Ben Nevis. 

 It is found, however, that when we use this table to reduce individual readings of the 

 Ben Nevis barometer to sea-level, these reduced values may differ from the observed 

 sea-level pressures in Fort- William by six or even eight hundredths of an inch above 

 -or below, and the two barometers frequently differ by one or two hundredths of an inch. 

 As the Fort- William barometer is only 42 feet above sea-level, no appreciable error can 

 be introduced in reducing it, and we must look for the origin of these discrepancies to 

 the reduction of the Ben Nevis barometer. As a general rule, during cyclonic weather, 

 and when the difference of temperature between Ben Nevis and Fort- William is greater 

 than usual, the reduced Ben Nevis barometer comes out lower than that at Fort- 

 William ; while in anti-cyclones, or when the difference of temperature is small, it is 

 higher.* In a paper published in the Journal of the Scottish Meteorological Society, 

 an abstract of which is given in this Appendix (see p. 506), it is suggested that the cause 

 of the discrepancy in anti-cyclones is that the temperature taken, namely, the mean of 

 the Ben Nevis and Fort-William temperatures, is not the true average temperature of 

 the air between the levels of the stations in such weather, and that this explains the 

 difference between the reduced barometers. 



When we consider the relation of the two barometers at sea-level at different hours 

 of the day, a difference is found between them similar to that which appears in con- 



* See " Relations of Pressure and Temperature," by Dr Buchan, page 496. 

 ROY. SOC. TRANS. BDIN. — VOL. XLII. 3 U 



