800 MR J. Y. BUCHANAN ON THE 



ture. The mean dew-point for each month is given for clear weather, and the excess 

 over it of the mean monthly temperature in foggy weather is given. It is only in 

 June and July that the mean temperature of the air in foggy weather is under the mean 

 dew-point in clear weather. As the air is always completely saturated in foggy weather 

 on Ben Nevis, the temperature of the air is also its dew-point. 



Summary. 



The paper deals only with observations made at the observatory on the summit of 

 Ben Nevis. The position of the observatory is lat. 56° 48' N., long. 5° 0' W., and 4407 

 feet or 1343 metres above sea-level. Greenwich mean time is used at the observatory. 

 The apparent time is always earlier than that shown by the clock, and the amount 

 varies from three to thirty-five minutes. A table is given at page 785 to facilitate the 

 conversion of g.m.t. into local apparent time. The length of the day varies from six 

 and a half hours at mid-winter to seventeen and a half hours at mid-summer. A table 

 is given at page 786, showing the dates when the length of the day is an exact number 

 of half hours. 



The observations are made every hour, and by eye. The period covered by this 

 paper is from 1st January 1885 to 31st December 1897, or thirteen complete 

 years. 



The weather on Ben Nevis is characterised by great prevalence of fog or mist. The 

 principal object of this paper is to select dates of continuous foggy weather and of 

 continuous clear weather, and to discuss them separately. A spell of continuous foggy 

 weather means three or more consecutive days during which fog has been logged at 

 every hour. Clear weather is comparatively so rare -that the minimum length of spell 

 was taken at twenty-four hours, and the characteristic of the weather is that fog shall 

 not be logged once during the spell. At page 784 a table gives a summary of the 

 material used. It amounts in the aggregate to about one-third of the total available, 

 without distinction of weather. Full details of the method of selection are given at 

 page 782. The data for pressure, temperature, tension of aqueous vapour, rainfall, 

 wind, cloud, and sunshine were sorted out, and fill eighty-four note-books. These will 

 be deposited with the Scottish Meteorological Society. 



The information contained in these note-books has been concentrated into the thirty- 

 four Tables which form the main part of this paper, and they are illustrated by the 

 eight Plates of curves at the end. The Tables and the Plates explain themselves. It is 

 sufficient to call attention to one or two salient features. 



It must be remembered that, for the purpose of this paper, clear weather means only 

 absence of fog, and predicates nothing with regard to presence or absence of cloud over- 

 head. 



