146 REPORT 1891. 



lines are suddenly deflected upwards. Now in such cases the de- 

 pression of the barometer is about three times as great as that which occurs 

 with an equally strong wind from other directions, and indicates clearly 

 the formation of a restricted region of low pressure around and outside 

 the observatory. Another curious and highly intei-esting result observed 

 with other directions of the wind is that the reduced high-level baro- 

 meter exceeds the I'ednced low-level barometer when the wind blows at 

 the rate of about ^j miles an hour. This increased pressure accompanying 

 wind rising up the slope of the hill may perhaps explain the small clear 

 space immediately on the top of a hill, otherwise cloud-topped, and the 

 very different force of wind on the two sides of a ridge lying about a 

 right angle to the direction of the wind. 



An examination has also been made of the relations of differences of 

 temperature at the two observatories to differences of the sea-level pressures 

 at the same hours. During the ten months examined the temperature 

 differences have ranged from the high-level observatory showing a tem- 

 perature 26° lower to a temperature 6° higher than the temperature at 

 Fort William at the time. A comparison has been made by sorting the 

 differences into two-degrees amounts, and instituting a comparison only 

 on those cases when the strength of the wind at either of the observatories 

 did not exceed 2G miles an hour. 



The following show for each two-degi'ees difference of temperature 

 the difference between the reduced barometer of the top and the barometer 

 at Fort William, the plus sign indicating that the top barometer was the 

 higher, and the minus sign that it was the lower of the two : — 



The broad result is this, and it is clear and explicit, when the higher 

 observatory has the higher temperature, and when the differences of 

 temperature are sm.^11, then the reduced pressure at the top of the moun- 

 tain is the greater of the two ; but when the differences of temperature 

 are large then the reduced pressure at the top is the less of the two. 

 The regular progression of these figures show that what is substantially 

 a true average has been obtained. The result, which is altogether unex- 

 pected, raises questions of the greatest importance, affecting the theory 

 of storms, the effect of vertical movements of great masses of air on the 

 barometric pressure which accompanies cyclones and anticyclones, and 

 the necessity there is for some accurate knowledge of the absolute 

 amounts of aqueous vapour at different heights in the atmosphere under 

 different weather conditions. Ben Nevis, with its two observatories, one 

 at the top, the other at the foot of the mountain, would, with a third 

 halfway up the hill, afford unique facilities for the prosecution of this 

 all-important hygrometric inquiry, which would, however, require 'con- 

 siderable additions, for the time it is carried on, to the observatories' 

 present appliances and staff. 



