514 



MR OMOND ON THE 



trasting cyclonic and anti-c}~clonic weather. During the day-time the difference of 

 temperature between the two Observatories is greater than the average for the whole day, 

 and the Ben Nevis barometer, when reduced, is, as a rule, lower than the Fort- William 

 barometer. At night the difference of temperature is less, and the Ben Nevis barometer 

 is the higher of the two. Further, this diurnal range in the difference of the two 

 barometers at sea-level does not merely appear when long averages are taken, but is in 

 evidence on almost any day on which strong winds do not vitiate the readings by 

 inducing pumping, and any change in the character of these differences marks a change 

 of weather conditions, which may ultimately be found to be of value in forecasting 

 changes in the type of weather. In order to eliminate the effects of wind, I have, out 

 of six years' records (1891-97, omitting 1893), taken only those days on which the 

 wind at Ben Nevis did not exceed force 2 (or 12 miles an hour) at any single hour of 

 the 24. Days with somewhat stronger winds might have been included without 

 impairing the accuracy of the Ben Nevis barometric readings, but taking only days on 

 which the wind was slight, simplifies the problem by practically eliminating the horizontal 

 movement of the air, and leaving only expansion, contraction, and convection currents 

 as factors to be dealt with. By thus selecting the calmer days, we also get on the 

 whole the fine or anti-cyclonic weather, though a few cases naturally come in of calm 

 weather in low pressure areas. The total number of such clays in the six ye # ars dealt 

 with was 267, or about 12 per cent, of the whole, and their distribution over the 

 different months and years is shown in the accompanying table : — 



Number of Days in which the Wind Force on Ben Nevis did not exceed 2 at any hour of the day. 





1891. 



'92. 



'93. 



'94. 



'95. 



'96. 



'97. 



Totals. 



Seasonal totals. 



January, . 

 February, . 



5 



2 



3 



2 







 1 



2 

 2 



4 

 



5 



2 



19 

 9 



Winter, 38. 



March, 



April, 



May, 



5 

 11 



7 



3 

 5 









3 



2 

 11 





 3 

 4 





 2 

 6 



1 



4 

 7 



12 

 27 

 35 



Spring, 74. 



June, 

 July, 

 August, 



9 



10 

 G 



4 

 4 



4 



... 



9 

 6 



7 



7 

 7 

 5 



6 



7 

 5 



5 

 6 

 5 



40 

 40 

 32 



Summer, 112. 



September, 



October, 



November, 



2 



5 

 4 



1 

 1 

 1 



... 



8 

 3 

 



1 







1 



3 

 2 

 3 



4 

 1 

 3 



19 

 12 

 12 



Autumn, 43. 



December, 



2 



1 



... 



2 



2 



2 



1 



10 





Total, 



68 



29 



... 



52 



34 



40 



44 



267 





Owing to the small number of days available, especially in the winter months, they 

 have been grouped in four seasons of three months each, viz., spring, March, April, 



