144: EEPOET — 1891. 



post and rope seem running over -with water, though, looking out at the 

 weather, one has no idea it is nearly so wet. 



Snfi&cient observations have been made to show a well-marked diurnal 

 variation in the numbers of dust particles. The following are the tri- 

 honrly results for March, April, and May, 1891 : — 



The daily minimum thus occurs when the daily strength of the wind is 

 greatest, and also the descending current, down the mountain, and the 

 maximum when the wind is least strong and the ascending current up the 

 mountain strongest. 



Mr. R. C. Mossman has communicated a paper to the Scottish 

 Meteorological Society on the cases of silver thaw at the Observatory, 

 which will appear in next issue of the Society's Journal. From 1885 to 

 1890 there occurred 198 cases, lasting in all 873 hours — that is, cases in 

 which rain froze as it fell. The maximum frequency is from November 

 to March. It occasions, as may well be supposed, much inconvenience 

 -and discomfort to the observers. 



The chief point established by Mr. Mossman is that the distribution 

 of pressure over Western Europe is at the time always substantially the 

 same. The daily weather charts show that on these 198 days the distri- 

 bution of pressure was for the Ben cyclonic on 137 and anticyclonic on 

 ■61 days. In anticylonic cases a cyclone is off the north--«'est coast of 

 Norway, while the anticyclone stretches away over the south of England 

 ^nd Ireland. In cyclonic cases Ben Nevis is clearly within the area of 

 low pressure, the centre of which again is off the north-west coast of 

 Norway, while the anticyclone is removed farther to southward over the 

 Peninsula. Hence the value of this phenomenon in forecasting weather. 

 The average duration is 6 hours in winter and 3 in summer. The longest 

 continued was 41 hours on January 3-4, 1889. The lowest temperature 

 .at which it has occurred was 18°'0, but nearly in all cases the occurrence 

 takes place shortly before a thaw. 



During the past year the uni-emitting attention of Dr. Buchan has 

 been given to the examination and discussion of the hourly observations 

 •of the two observatories. The discussion includes the ten months ending 

 May, 1891. 



In entering on the discussion it quickly became apparent that the 

 influence of high winds on the barometer was the first inquiry calling for 

 serious attention. The depression of the barometer during high winds 

 was plainly so serious as to render the examination of many questions all 

 but a hopeless task until some approximation was made to the values of 

 these depressions for different wind velocities. 



Fortunately the two observatories present the conditions favourable 

 for this investigation. They are so near to each other as to form vir- 

 tually but one observatory, the barometer at the top being in a building 

 exposed to winds of all velocities up to at least 150 miles an hour, whereas 

 the other barometer is in a sheltered building, where light winds prevail 



