90 
The Lecturer directed the Club’s attention to the atmos- 
pheric disturbances which more nearly concern ourselves — 
those prevailing around the British Isles. The Lecturer 
first shewed, by means of a map, the prevailing winds which 
blow in our part of the northern hemisphere. Unsettled 
and irrational as our climate appears to be, it may yet be 
explained and fairly accurately foretold if we take the trouble 
to avail ourselves of the daily information supplied to us 
by the Meteorological Office. This useful institution issues 
daily a chart, which is published in The Times, several 
of which charts, shewing the weather conditions in the 
British Isles from day to day, the Lecturer put upon the 
screen. The charts consist of a rough map of our islands 
with the seas about them. They shew a number of dotted 
lines called “ isobars,” indicating that at places along any 
such line the barometrical pressure is the same, and the 
height of the barometer along each line is shewn by figures 
at the end. The temperature and the strength and direction 
of the wind at various places are also shewn. All these 
records are obtained daily by telegraph, and enable us to 
get simultaneous readings from all parts. 
As might be expected, the tendency is for air to move 
from a district where the pressure is high to a district where 
it is low, and the speed with which the motion takes place 
depends upon the distance and difference in barometrical 
pressure between the two districts. Where the isobars are 
shewn near together, we get, as we should expect, a rapid 
wind from the higher to the lower, and where they are far 
apart the wind is light. 
The charts also shew what are known as cyclones and 
“ anticyclones.” The cyclones are centres of low pressure, 
i.e., places in the atmosphere where the pressure is lower 
than at any surrounding spot, and consequently the air 
from all round comes in to fill the pit. In the northern 
hemisphere, for the reason given respecting the direction of 
the trade-winds, wind from the north falls to the west of 
its object, and wind from the south is carried eastward, 
whereby is set up an anti-clock-wise circular motion of the 
air around the centre of depression. If we observe the position 
of these cyclones in the weather charts and their motion 
across our islands, we have an explanation of what might 
otherwise seem mere haphazard changes of wind. As a 
cyclone, for instance, moves eastward we get, it it passes o\ er 
us, first southerly winds with a falling barometer, then com- 
parative calm, and then northerly winds and a rising glass. 
An anticyclone is a centre of high pressure, and in this case 
