144 CYCLONES. 
that would only gradually take it towards our air-pump (see Fig. 
1); i fact, a whirlpool would be established in the air. Now 
let us look at the real weather-map again. (Plate I., Fig. 3). 
Here we notice going on exactly what we imagined just now. The 
arrows which mark the course of the wind, you will notice, are all 
pointing round the circle—not quite parallel to the lines of equal 
barometric pressure, but a little inwards, showing that the wind 
is blowing, not only round, but, also, gradually towards the centre. 
The air-pump I must leave to your imagination. That there is 
something of the sort is shown, however, by the fact that these 
circular systems of pressure exist for days; and we have seen 
just now that if there was not something at the centre to remove 
the air as fast as it blew in, the whole system would quickly 
cease to exist, owing to being filled up to an even pressure with 
air from the outside. Now what becomes of the air which is 
constantly blowing in towards the centre? We see that it must 
escape somehow. As we know the air blows in at the sides, 
this leaves only the top and bottom of our circle for its exit; the 
bottom is, of course, impossible, therefore upwards is its only way 
of escape ; and this, in fact, is what happens. Observation shows 
that the air flowing in towards the centre, there ascends into the 
higher regions of our atmosphere. 
This system of air pressure and wind motion, which I have 
just described to you, is what is known as a Cyclone, or ‘ Depres- 
sion.” A Cyclone, so far as we have gone at present, is, then, a 
gigantic eddy, or whirlpool, in the air, often many hundred miles 
across, at the centre of which the air pressure is always lower than 
at the sides; and towards this centre the wind blows in a spiral 
course, and afterwards is carried upwards into the higher regions” 
of the atmosphere. 
We will now examine more minutely the properties of these 
Cyclones. In the first place, if a series of weather-maps are 
examined for a number of consecutive days (by referring to the 
daily weather-maps in the Zvmes, for instance), it will be found 
that these Cyclones are hardly ever stationary over any one portion 
of the earth, but are almost always moving at a considerable 
