492 DR BUCHAN ON THE 



Transactions of the Royal Society of Edinburgh* and the readings at Fort- William, 

 in the usual way. The differences of the two reduced readings were then entered 

 in columns headed 0, 1, 2, 3, etc., and according to the wind force at the Ben 

 Nevis Observatory at the time. The following Table gives the mean differences 

 for each wind force for each wind, and the number of observations from which mean 

 differences have been calculated. The comparison for the ten months was made from 

 the hourly observations at both Observatories from August 1890 to May 1891. But 

 since this period gave too few observations for the higher wind velocities for good 

 averages, the observations from January 1885 to July 1890 were utilised for the five 

 hours of the day at which corresponding observations were made at Fort- William. 

 Only the wind forces from 5 to 11 have been thus utilised, and the results are incor- 

 porated with those for the means of the ten months. In all, upwards of 8000 double 

 observations made at the two Observatories have been reduced to sea-level for these 

 comparisons. 



The following summarizes the results, showing the depression of the barometer with 

 each wind velocity : — 



Beaufort's Scale. 

 to 12. 



Equivalents in Miles. 

 Per Hour. 



Baro. Depression 

 Inch. 







1 

 2 

 3 





 6 



12 

 21 





-o-ooi 



-0-004 

 -0-005 



-o-oio 



4 

 5 

 6 



7 



30 

 39 

 49 

 60 





-0-014 

 -0-026 

 -0-035 

 - 0-050 



8 



9 

 10 

 11 

 11 to 12 



72 



84 



97 

 112 

 130 and 



higher 



- 0070 

 -0-104 

 -0-126 

 -0-150 

 -0-170 



Thus in calm weather, the two reduced barometers are practically the same, but 

 with every increase of wind the depression of the barometer steadily augments. It is 

 not till a velocity of more than 20 miles an hour is attained that the depression amounts 

 to one hundredth of an inch. At 60 miles an hour, it is 0*050 inch ; at 84 miles, 

 0'104 inch ; and at 130 miles, 0'170 inch. 



This depression of the barometer is no doubt occasioned by the wind drawing out 

 the air from the room where the barometer is hung, as it rushes past the Observatory, 

 thus producing a partial vacuum and consequently a lower pressure. If a window or 

 door is opened on the side of the room exposed to the wind, the readings of the 

 barometer are thereby raised ; whereas on the lee side of buildings, in rooms connected 

 therewith, and in rooms with chimneys, the barometric readings are lowered. Now, as 

 the barometer of the Ben Nevis Observatory is hung in a room, with the usual chimney, 



* Trans. Roy. Soc. Edin., vol. xxxiv. pp. b:., lxi. 



