This investigation was undertaken with the view of ascertaining how the mean hourly values of the barometer differed on days in which radiation, solar and terrestrial was great from days in which it was interfered with by clouds. A preliminary examination of the barometric readings at the Ben Nevis and Fort-William Observatories showed that there was a well marked difference between the diurnal fluctuations in these two kinds of weather; and the investigation was thereafter extended to several other places in the northern hemisphere. The materials for this work are only available from comparatively few places, it being necessary to have before us the hourly readings of the barometer for every day, and also an hourly record either of sunshine or of cloud. These two separate data have not been published for any place in the British Islands except Ben Nevis and Fort-William.

The publication by the Royal Society of Edinburgh of the hourly observations made at the observatory on the summit of Ben Nevis along with corresponding observations at Fort-William at the sea level is an important contribution to meteorological science. The study of this work reveals at once the important characteristic of the Ben Nevis Observatory. It is situated on a true peak, and that the highest point in the British Islands. Its height above the sea is such that it is usually enveloped in cloud, and its records are for the most part those of an observatory established in the clouds, and in an atmosphere habitually saturated with moisture.

In 1889 the Scottish Meteorological Society decided on making observations on the dust in the atmosphere at their Observatory on Ben Nevis. Having prepared the plans for the apparatus to be used for counting the dust particles, I visited the Observatory on the 1st of August of that year, and, after consulting with Mr Omond, decided that the tower of the Observatory was the best place for fitting up the instrument. The apparatus was afterwards made by White of Glasgow, and fitted up in the Observatory on 5th June 1890.

In Part I. the influence of high winds on the barometer is examined for all directions of wind taken together; but in Part II. the different directions of wind are treated separately. The question of the effect of wind on the readings of the barometer was first examined by Sir Henry James in a paper read to the Royal Society of Edinburgh on 15th March 1852. The observations were made during the succession of gales from the south-west which occurred in January and February of that year at Granton, near Edinburgh, with an aneroid barometer, laid horizontally in succession on the table of a room in a cottage, on the seat of an open summer-house, and on the surface of the ground close to the summer-house, all at the same level.

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 observations 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.

Noting the direction of the wind is part of the regular routine of observations at the Observatory on the summit of Ben Nevis. Owing to the frequency of frozen fogs in winter, anemometers are practically useless, but the record is made complete enough for many purposes by the hourly observations; at each hour of the day and night the wind, at the time of making the observation, is noted, and the number of entries of each wind direction in the day, month, or year must very nearly represent the relative frequency of such winds for that time. Table I. is a summary giving the number of times each wind is observed in each month of the year on an average of six years (1884–89) expressed in percentages; thus in January, out of the 744 hours of the month, there are 120 hours or 16·1 per cent, of N. wind. For the year the most frequent wind is N., next come S.W., S.E., and S., all about equal, and the least frequent is E. N. is the point of greatest frequency in all months except May, when S.E. takes its place; in May 1889 there were only 15 hours in the whole month with N., N.W., or N.E. winds, and during three-fourths of that month the air was moving from the S. or S.E. This was, of course, an exceptional month, but in the six years there are only two (1887–88) in which N. was the most frequent wind in May.

During almost every winter anti-cyclone, with its accompanying cold but calm and fair weather, the temperature at Fort-William is, for part of the time, nearly as low or lower than that on Ben Nevis, 4400 feet above it. The average difference of temperature is 16° Fahr.; Ben Nevis being that much colder, or about 1° for every 270 feet. But in anti-cyclones this normal difference of temperature is widely departed from, the records showing that Ben Nevis has been 17° warmer instead of 16° colder than Fort-William, a departure of 33° from the normal; and it has frequently been 5° to 10° warmer for a day or two at a time. These data only give the temperatures on the summit of Ben Nevis, and at the sea-level five miles away they give us no information as to what it is in the intermediate air. That there is something unusual in this 4400 feet of atmosphere is hinted at by two facts,—first, with this abnormal temperature arrangement we always have very dry air on the summit, and very damp air at Fort-William; second, the difference of the barometer readings at top and bottom is less than the normal difference for the temperature. The former is an inversion of the ordinary humidity conditions—usually the air on the summit is saturated, or nearly so, with moisture, and at Fort-William dry; the latter, the barometric differences, require a few words of explanation to show what exactly they indicate, and how they bear on the question of temperature.

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.