ON METEOROLOGICAL OBSERVATIONS ON BEN NEVIS. 145 



generally, so that this barometer may be regarded as recording the 

 true pressure of the atmosphere. This was more exactly secured 

 in making comparisons of the two barometers by selecting only those 

 cases when winds at the Fort William Observatory were light. As 

 stated by the Committee in previous reports, the observations of the force 

 of the wind are estimations on a scale of to 12, the equivalent of each 

 figure of the scale in miles per hour having been carefully determined 

 by Mr. Omond by means of Chrystal's anemometer. The barometric 

 observations at the two observatories were reduced to sea-level hour by 

 hour, and the differences plus or minus were entered in columns repre- 

 senting the different wind forces at the higher observatory. The following 

 is the result of the comparison : — 



Thus in calm weather the two reduced barometers are practically the 

 same, but with every increase of wind which sweeps past the hio-her 

 observatory, the depression of the barometer inside steadily augments. 

 It is not till a velocity of more than 20 miles an hour is reached that 

 the depression amounts to one-hundredth of an inch. At 57 miles it is 

 0-050 inch, at 11 miles 0-104 inch, and at 99 miles 0-150 inch. In 

 forecasting weather it will be necessary to keep this effect of high winds 

 on the barometer constantly in mind, with the view of arriving at a 

 better approximation to the geographical distribution of pressure at the 

 time the forecasts are being framed. 



These results are for all winds grouped together irrespective of their 

 direction. The next inquiry grouped the winds according to their direc- 

 tion to sixteen points of the compass. During the time under examination, 

 all the very high winds were from E.S.E. or S.E., these being the direc- 

 tions in which the wind blows freely along the slopes of the mountain to 

 the observatory. In 11 cases the wind from these directions attained a 

 Telocity of 100 miles an hour or more, and the reduced barometer of the 

 high-level station read about one-sixth of an inch lower than the baro- 

 meter of the low-level observatory. In no other of the 16 directions was 

 there, during the ten months, a higher velocity than 62 miles an hour 

 observed, and indeed in the directions E., E.N.B., KE., K, N.W., and 

 W. the observed velocity was never greater than 29 miles an hour. 

 With these northerly winds the observations at the top of the mountain 

 indicate a much lower speed than that which, from the drift of the clouds, 

 IS seen to be reached at a comparatively small height above the top of the 

 J3en The cause of this comparatively calm state of the air immediately 

 on the top is the impact of the air on the face of the tremendous cliff, 

 close to the top of which the observatory is built, by which the stream 



1891. .7 ' J 



