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.