INFLUENCE OF HIGH WINDS ON BAROMETER AT BEN NEVIS. 491 



practically no advance has been made in this branch of meteorology. Various arrange- 

 ments have been proposed, but none of them can be regarded as satisfactory, with a 

 view of arriving at a knowledge of the actual pressure of the free atmosphere during 

 high winds. The difficulty consists in finding a perfectly unscreened position for the 

 barometer, and securing at the same time that the wind, brushing past the small 

 openings connecting the mercury of the cistern with the air outside, will not partially 

 lower the pressure on the mercury in the cistern, and so render the instrument no 

 longer indicative of the true pressure of the free atmosphere. 



In carrying out, during the past year, the instructions of the directors of the Ben 

 Nevis Observatory to discuss the observations made at the High and Low-Level 

 Observatories, 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 large 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. 



Now, since the horizontal distance of the High and Low-Level Observatories is only 

 about 4 miles, it follows that the two may virtually be treated as one so far as the 

 geographical distribution of pressure is concerned. But the Observatory at the top of 

 the mountain is peculiarly exposed to high winds, which are occasionally so violent that 

 the observers must be roped together on going outside to make the observations ; and 

 it not unfrequently occurs that very strong winds prevail, while over the surrounding- 

 low country there are only calms and light winds. On the other hand, the Low-Level 

 Observatory at Fort- William is in a comparatively sheltered position, and winds blowing 

 over the Observatory at rates exceeding 30, or even 20, miles an hour are of infrequent 

 occurrence. Thus, then, these two Observatories present the conditions so essential to 

 this inquiry, viz., one of the barometers is in a building exposed to winds of all 

 velocities up to at least 150 miles an hour, whereas the other is in a building where 

 either calms or only light winds at the time prevail — so that this barometer may be 

 regarded as recording the pressure of the free atmosphere. It was, therefore, resolved 

 to institute a comparison between the sea-level pressures of these two barometers, 

 employing only those cases when winds at the Fort- William Observatory were light. 



The scale used on Ben Nevis for the observations of the force of the wind is a 

 modification of Beaufort's scale, to 12. Much attention has been given to ascertain 

 the wind's rate in miles per hour, corresponding to each of the figures of Beaufort's 

 scale. For this purpose, a modification of Robinson's anemometer was designed by 

 Professor Chrystal for the Observatory ; and during the times the instrument has not 

 been frozen up in a thick covering of ice, numerous comparisons have been made. 

 These have been discussed by Mr Omond, and the equivalents in miles per hour been 

 determined. (See Introduction, page 5.) 



The reductions of the barometric readings on the top of Ben Nevis to sea-level have 

 been made by a table prepared for the purpose, given in a former volume of the 



