The result of that is that there
is less _pressure_ at A--"
"Is it fine then?"
"I've explained already what happens then. The air from B--"
"Do we begin all over again now?"
"More or less, yes."
"So that at this place, A, it's always raining or just going to rain?"
"Yes, if it starts by being hot there, as it did just now, I suppose it
is."
"What happens if it starts by being cold?"
"It rains. I've explained that. The cold air can't contain so much
moisture--"
"Don't begin that again. What about B? Is it any good going there? We had
frightfully high pressure there at one time."
"Yes, but it rains so much at A that more and more air rushes from B to A
to fill up the gap caused by the air rising on account of the heat
generated by the rain falling, and very soon you get frightfully low
pressure at B--"
"Is it fine then?"
"No, it rains."
"You surprise me. But suppose it had started by being low pressure at B?"
"Why, then of course it would have been raining the whole time at B."
"Where would A have got its rush of air from then?"
"From the place C."
"Is it fine there?"
"No, it's raining. It is like B was after the air rose at A."
"Oh. Then whatever happens at these places, A, B and C, it _must_ rain."
"More or less, yes. More really."
"Are there any more places? I mean, if I am at A where ought I to go?"
"There is a place, D--"
"What happens there?"
"Conditions are favourable for the formation of secondary depressions.
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