For a few years readings of a Black Bulb in vacuo thermometer formed part of the regular hourly routine work at the Observatory on Ben Nevis during the summer months, and a summary of two years' observations, for 1890 and 1891, is given in this paper. The instrument consists of the usual vacuum chamber, a spherical glass globe about 2½ inches diameter, with inside, centrically, a thermometer whose bulb and an inch or so of stem are coated a dull black. The thermometer is an ordinary, not a maximum Fahrenheit thermometer, and therefore gives the Black Bulb temperature at the instant of reading only. It was exposed on a wooden stand in a horizontal position, with the bulb projecting beyond the frame, and pointing south, about 4 feet above the broken stones which form the hill top, and was read immediately after the shade temperature during daylight hours.

The accompanying table shows for the thirteen years, 1884 to 1896 inclusive, the number of hourly observations at which the force of the wind was noted as above 6, on the Ben Nevis scale of 0 to 12. This is equivalent to all cases when the velocity of the wind exceeded fifty miles per hour. The table gives the totals for each hour of the day of the thirteen Januarys, thirteen Februarys, etc., in this period, with at the foot the sum for each hour, which is the total number of cases observed at each hour during the thirteen years. At the right hand of the table is a column giving the monthly totals (the sum of the twenty-four hourly values); the gross total of the whole, amounting to 3405 cases in the thirteen years, gives an average of 262 hours, or about eleven days, per annum.

The phenomenon of silver thaw,—that is, rain falling when the temperature is below the freezing-point and congealing when it falls,—is of somewhat common occurrence at the Ben Nevis Observatory. A prolonged fall of silver thaw occasions considerable inconvenience to the observers; the rain freezes on their coats, gloves, and even on their faces, while everything outside becomes covered several inches thick with solid ice. But the most serious effect is the choking of the louvres of the Stevenson screen where the thermometers are, necessitating frequent changing of the screens.

The average variability of temperature at any place is obtained by taking the difference of temperature at the same hour on successive days, and taking the mean value of this difference irrespective of sign. With a view to ascertaining whether this variability varied with the hour selected for comparison, the hourly temperature records at four places have been examined, and the day to day change of temperature at each hour noted. The places are—the Ben Nevis Observatory, the Fort-William Observatory, the Hong Kong Observatory, and the Arctic Station at Lady Franklin Bay, lat. 81° 44′ N., long. 64° 45′ W. At each place one year's record was taken, which, though too short a time to give a true mean, yet gives a fair approximation towards it. The mean values for each hour of the twelve months at the four stations are given in the accompanying tables, the highest value in each month being put in heavy type and the lowest in italic.

This great frost commenced on 28th December 1894 and terminated in its intense form on 20th February 1895, thus lasting 54 days. The greatest cold occurred from the 6th to the 20th of February. During this latter period pressure was higher in the north than in the south of Scotland, and the winds were consequently easterly or north-easterly, but light in force. Taking the two months, January and February, together, the mean temperature was more than 10° below the average at many inland places, but on the west coast it was only about 5° lower than usual. There are records of temperature in the north-east of Scotland, at either Inverness, Gordon Castle or Aberdeen for the last 132 years, and in all this time there was no winter with two such cold months as January and February 1895 at these places.

The more prominent results of Mr Wragge's work were discussed by Dr Buchan in a paper on the “Meteorology of Ben Nevis,” published in the Journal of the Scottish Meteorological Society (vol. vii. p. 4). The humidities at the intermediate stations on the hill-side have not, however, as yet been discussed. One reason for this was that, until Dr Herbertson and Mr H. N. Dickson carried out their researches on hygrometry at Ben Nevis Observatory, it was not known how far Glaisher's tables could be regarded as reliable at high-level stations. Their experiments, however, showed that for temperatures above 40°, Glaisher's factors could be used except when the air was excessively dry. As Mr Wragge's observations were taken between June and October, when the temperature was relatively high and the air nearly saturated, Glaisher's tables have been employed to compute the humidity and vapour pressure from the dry and wet bulb readings.

When the Observatory on the summit of Ben Nevis was opened in 1883, arrangements were made with Mr Colin Livingston, of the Public School, Fort-William, whereby observations were made five times a day at the Schoolhouse for comparison with those on Ben Nevis. These observations, from December 1883 to December 1887, were published in vol. xxxiv. of the Transactions R.S.E., along with the Ben Nevis records; and three years more, up to December 1890, are given in this volume (see Introduction, p. 7). The Fort-William Observatory was opened in August 1890, but Mr Livingston kindly continued his observations till the end of 1891, so as to allow of a comparison between these two stations, which was the more desirable as Mr Livingston's observations had been used for the original determinations of the differences of pressure and temperature between Ben Nevis and sea-level, and any discrepancy between the observations made at the Schoolhouse and at the Observatory would necessitate a readjustment of these constants.

In correcting the reading of a barometer to the freezing-point, it is customary to ascertain the temperature of the instrument by observing a thermometer attached to the brass frame, but it is doubtful how far this attached thermometer indicates the temperature of all parts of the barometer under the conditions in which most observations are made. In a Fortin barometer the level of the mercury in the cistern is adjusted to a fixed point at each observation, and we have only to consider the expansion with heat of the mercury and of the brass scale.

The mean hourly values of the different meteorological elements given in this volume are the simple arithmetical means (or sums) of the observations; but in the tables, which show the departure of each hourly value from the mean for the whole day, a correction has been applied to eliminate any general rise or fall which may take place throughout the whole day, and thus to more nearly show the true diurnal effect independent of changes of longer period than twenty-four hours.

A table for reducing the barometric readings at Ben Nevis Observatory to sea-level was given in the former volume of observations, and is reprinted here. The table was compiled by direct comparison of the barometric readings at the Ben Nevis Observatory and at the Public School, Fort-William, and consists of the average difference between these barometers, both being reduced to 32°, and the Fort-William one brought to sea-level, for each tenth of an inch of pressure from 30·9 to 27·4 inches, and for each degree of temperature from 16° to 75°. Any missing values were filled in by interpolation, and the whole smoothed by cross averaging. A full description of the preparation of the table, which is regarded as of a provisional character, will be found in the former volume. (See Transactions Royal Society, Edinburgh, vol. xxxiv. p. xxv.)