142 CYCLONES. 
that are separated from each other by ;2; or o*2 of an inch (see 
Fig. 3). 
Now let us consider the direction of the wind, which has also 
been telegraphed to the central office in London. ‘This direction 
of the wind is marked on the map by the arrows, which are drawn 
pointing in the same direction that the wind is blowing (see Fig. 
3). To give a representation also on our map of the strength 
with which the wind is blowing at the time of observation, the 
wind-arrows are differently formed, as you will see. For instance, 
a light breeze is marked by an arrow with half a head to it; a 
strong breeze, by an arrow with a complete head; a gale is shown 
by one feather on each side the arrow; a strong gale by two 
feathers, and so on. 
We see, then, by looking at this synchronous weather-map an 
accurate chart of the weight or pressure of the air over all this 
part of the world, and also the direction and force with which 
the wind was blowing at the time of observation. Now let us 
look more closely at one portion of the map (Fig. 3), where 
these black lines that show the pressure or weight of the air 
are in a circular form. If we look at the figures which show 
the height of the barometer on the particular lines on which they 
are placed, we find that near the centre of the circle the baromete! 
stands at 28°7; at the next line the height is 289, that is to say, 
=, of an inch higher, therefore the pressure of the air is higher 
on the second line than on the first. The next line shows the 
pressure is still 2, higher, that is to say, the weight or pressure of 
the air is still greater; and so on you see the height of the 
barometer zzcreasing as we go away from the centre of this circle, 
So you see, in this circle we have a certain portion of the world’s 
surface over which the weight or pressure of the air is gradually 
increasing from the centre to the outside. Now, what would be 
natural to expect if we managed to build up with air a construction 
of this sort, and after surrounding it with -an unlimited amount 
of air outside at an even pressure, left it to itself? (Fig. 1 
represents an imaginary cyclone of this nature.) Why, 
of course, as the outer layer is heavier than the next 
