92 Proceedings of Royal Society of Edinburgh. [sess. 
The following summarises the results, showing 
the barometer with each wind velocity : — 
the depression of 
Miles per hour. 
Baro. Depression, 
inch. 
Miles per hour. 
Baro. Depression, 
inch. 
0 
-0*001 
50 
-0 035 
5 
-0*004 
63 
-0*050 
12 
-0*005 
83 
-0*070 
21 
-0*010 
96 
-0*104 
31 
-0*014 
108 
-0*122 
39 
-0*026 
120 
-0*150 
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 63 miles an hour, it is 0*050 inch ; at 96 
miles, 0T04 inch; and at 120 miles, 0*150 inch. The amount of the 
depression of the barometer is thus practically proportional to the 
velocity of the wind, from zero to a velocity of 120 miles per hour. 
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, door, and windows, these results may be regarded 
as applicable to the readings of barometers generally, since they are 
in almost every case suspended in situations similar to that of the 
Ben Nevis barometer. 
In a paper on the Mean Atmospheric Pressure of the British 
Islands, published by the Scottish Meterological Society ten years 
ago,* monthly and annual isobars are given for every two-hun- 
dredths of an inch of pressure. These isobars show a lower 
pressure over those parts of the country where the prevailing winds 
are stronger than elsewhere. It may now be regarded as probable 
that the curved courses taken by the isobars do not indicate any 
* Journal Scot. Meteorol. Soc., new series, vol. vi. p. 4-40. 
