ON METEOROLOGICAL OBSERVATIOKS ON EEN NEVIS. 113 



air from the valleys. Thin cirrus clouds floated above the hill all day, 

 and detached masses of fog were in the valleys. The next day the hill- 

 top was enveloped in mist, and thick cumulus clouds were observed over 

 Fort William with a smaller hourly variation both of temperature and 

 water-vapour, 



Mr. Marr continued the work at Fort William at Christmas, and 

 Mr. Herbertson in May, the latter having carried on the experiments 

 in the drier air of Montpellier during the early months of this year. 

 These data have served to extend the curves drawn from the figures 

 already obtained, the Montpellier results being found to agree well with 

 those made at Fort William where they overlapped. 



It is Mr. Herbertson's intention to continue these investigations in the 

 coming autumn and winter, more special attention being then given to 

 secure an increased number of simultaneous high and low determinations 

 of water-vapour. 



The inquiry into the hourly variation of pressure and temperature at 

 the observatories during days of clear weather on the one hand, and days 

 of fog or mist throughout on the other, has been prosecuted during the 

 year. On completing the hourly variations of pressure during each of 

 these two distinct types of weather at the Low Level Observatory at Fort 

 William on the same days at the top of the mountain, it was seen that 

 substantially the same sets of curves obtained in both situations. In 

 particular at both places the same extraordinarily high pressure from 

 about 6 P.M. to 2 a.m. occurs during days of fog or mist, or completely 

 clouded days, thus demonstrating the important role played by the 

 aqueous vapour of the atmosphere in the diurnal meteorological changes. 

 The point was considered to be of such importance as to warrant the 

 extension of the inquiry to another place where the climate, as regards 

 moisture, resembles in some degree that of Fort William. Trieste, at the 

 head of the Adriatic Sea, was selected for examination, particularly since 

 the hourly values for pressure and sunshine are published for this place. 

 The results for the three observatories are given in Tables III. to VIII. 

 of this Report. The times over which the inquiry extends are three years, 

 from August 1890 to July 1892 for the Ben Nevis Observatories, and the 

 three years 1888 to 1890 for Trieste. The results have been ' bloxamed,' as- 

 explained in our last Report (p. 284). 



The results of the investigation, so far as it has been carried, are, 

 broadly stated, these : —During fine cloudless weather the hourly curves, 

 of the three places are virtually congruent with their curves for the 

 whole of the observations, clear and clouded alike, except that the diurnal 

 phases of maxima and minima are more strongly pronounced, and the 

 evening maximum is continued for a shorter period. During foggy and 

 clouded weather each of the three places shows in the colder months 

 of the year the ordinary double maxima and minima of pressure fairly 

 well marked. It is quite otherwise in the warmer months, when at the 

 Ben Nevis Observatories the morning maximum is virtually obliterated, 

 and Trieste very greatly reduced. It is, however, the evening maximum 

 which shows the most surprising results. This is at all seasons at the 

 low-level stations, but in the warmer months it is the outstanding feature 

 of the curves. For May, June, and July the mean at the top is 0-018 

 inch, at Fort William 0023 inch, and at Trieste also 0023 inch. In 

 these months the maximum of this phase of the barometric curve occurred 

 either at midnight or shortly before it. 



The temperature curves for the Ben Nevis Observatory have also been 



189'1. 1 



