THE WEATHER 



surface of the liquid in the outer vessel, it does 

 not all fall out of the tube. We shall find that 

 inside the tube the mercury stands about thirty 

 inches higher than in the little vessel outside. 

 If we fix a scale to measure this height, we shall 

 see that the reading varies from day to day, 

 and in this way we observe something which 

 depends on the pressure of the atmosphere. 

 Now let us look again at the little map in the 

 newspaper. Besides the arrows showing the 

 direction of the wind, are some dotted lines, 

 marked 29-9, 30*1, 30-3, and so on. These lines 

 are drawn through all places at which the baro- 

 meter stands at the same height, the figures 

 showing the number of inches between the level 

 of the mercury inside and outside the tube of 

 the instrument. Now these heights measure 

 the pressure of the atmosphere, and therefore 

 by looking at the map we can see at once how 

 the pressure of the atmosphere, that is, the 

 weight of the air, varied over the west of Europe 

 at six o'clock the previous evening. 



If we look at these barometer lines when 

 the wind is blowing in a circle in the opposite 

 direction to the hands of a clock, we shall see 

 that the pressure is lowest towards the centre, 

 and higher the further we go outwards. This 

 circular swirl with a low-pressure centre is called 

 a * cyclone.' On the other hand, when the winds 

 are circulating in the clockwise direction, the 

 area of greatest pressure lies inside the circles 



(67) 



