XXVI METEOROLOGY OF BEN NEVIS. 



The sea-level corrections for the Observatory have been worked out from 

 the formula given by Laplace in the Mecanique Celeste in the form employed 

 by the Meteorological Council, and in general use by meteorologists in the 

 United Kingdom.* At an outside air-temperature of 45° '0, the correction for 

 height deduced from this formula is identical with the correction calculated 

 directly from the observations for all the sea-level pressures. But at lower 

 temperatures the correction from the formula steadily increases, and at an air- 

 temperature of 20° '0 and sea-level pressure of 30 000, it is 040 inch larger 

 than the correction empirically determined from the Ben Nevis observations ; 

 while, on the other hand, with air- temperatures higher than 45° - 0, the empiri- 

 cally-determined correction is the larger one. It was found that, when the 

 additions to the corrections in the Laplace table for air-temperatures lower than 

 45° were reduced by one-sixth, and the subtractions from the corrections as the 

 temperature rose above 45° were also reduced by one-sixth, the two tables 

 were virtually identical. The admirable position of the two stations at which 

 the effects of solar and terrestrial radiation are minimised, gives quite a special 

 value to the result thus obtained. The table must, however, be regarded only 

 as a provisional one, giving closer approximations to the true corrections for 

 height than has hitherto been possible. 



In preparing new isothermal and isobaric maps of the globe for the 

 " Challenger " Expedition Report on Atmospheric Circulation, these rates 

 of diminution of temperature and pressure with height, deduced from the 

 Ben Nevis observations, were adopted in the construction of the table of 

 corrections for height above mean sea-level up to 8000 feet for the different 

 air-temperatures and sea-level pressures that occur. The results of charting 

 from these tables afford the strongest corroboration of the great value in 

 practical meteorology and physical geography of this piece of work executed 

 from the data furnished by the Ben Nevis and Fort- William Observatories. 



The departures from the normal rate of diminution of temperature with 

 height, differ materially from each other from time to time. On reducing the 

 barometer at the top of the mountain to sea-level, using in the reduction the 

 mean of the temperatures at the top and the bottom of the Ben as the tempera- 

 ture of the air, it is found in a large majority of cases that the reduced pressure 

 closely agrees with the observed sea-level pressure at Fort- William. This 

 result is far from being restricted to those cases when the rate of diminution of 

 temperature with height is closely approximate to the normal. It also fre- 

 quently occurs when the difference of temperature at the two Observatories 

 either greatly exceeds, or greatly falls short of, the normal, occurring even 

 when the two temperatures are nearly the same, or the upper temperature the 

 higher of the two. The agreement of the reduced pressure to sea-level of the 



* Instructions in the Use of Meteorological Instruments, by R H. Scott, London, 1873, p. 80. 



