16 THE BOOK OF NATURE STUDY 



here is another cause of air movement. In some such way intro- 

 duce the subject of pressure, give an idea of the constant changes 

 of which the air is the seat, always trying to begin with direct 

 observation. 



To sum up, then we have to realise that whether we think 

 of our own small locality, or of the great world at large, there 

 are everywhere differences of temperature in the air, and there- 

 fore differences of pressure. These differences of temperature are 

 brought about in two main ways. First, they are due to the 

 fact that the different parts of the globe do not all get the same 

 amount of sunshine and therefore of heat. Second, we find that 

 land and water, dry ground and moist ground, even when subjected 

 to the same amount of heat, become unequally hot, and heat 

 unequally the layer of air above them. This is what we learnt 

 from plunging our thermometer into the pond. 



To illustrate the next point the analogy of the bicycle tyre may 

 well be utilised. Let the members of the class explain from their 

 own observations that much air can be squeezed into a bicycle 

 tyre when the arm of the pumper is strong, let them tell how the 

 air whizzes out when the valve is removed, or the omnipresent thorn 

 enters, and let this precede the familiar statement that air or any 

 other gas, when free to move, tends to move from a region of high 

 pressure to one of low. Let them describe how the air rushes out 

 faster at first when the tyre is full than later when it is beginning 

 to empty. Then choose a day of violent wind, and while this 

 howls round the chimney-pots, point to the low barometer, and 

 by judicious questioning get the class to see that the low glass, 

 the shrieking wind, mean that the air from some region of high 

 pressure is rushing in to fill the gap which the low glass indicates. 

 On a day of high pressure, point out that then there is relative 

 calm, the air merely streaming gently out of our area into a more 

 distant one where pressure is lower. Before any details are given, 

 however, be sure that the connection of differences of temperature, 

 of differences of pressure, and of wind is grasped. Every change 

 of wind means a change of pressure, every change of pressure means 

 a change of temperature, and this brings us back to the differences 

 in the amount of heat and to the differential absorption which start 

 the whole circulation. 



