treat it as though it were, in thinking out what happens.
Let us make believe that there are different layers, as in Fig. 20, copied from a beautiful picture at the Meteorological Office at South Kensington
Fig. 20.
(which you ought to visit some time, because you will find all sorts of interesting things there). The first layer takes us about six miles up, and may be called the layer of mountains. The highest mountain in the world (Mount Everest) reaches just about to the top, and you see how small the Eiffel Tower looks beside it. The highest aeroplane record is about four miles, so that the first layer is also that of aeroplanes.
A kite has been up a little higher than that. And a man in a balloon has actually been just into the second layer, but only just. So that our human experience of the air is practically confined to the first layer, and we knew nothing about the layers above it till quite recently. All our knowledge of them is due to the use of balloons, rather like toy balloons,
