there is more room for the air to move about. When that happens the
air at the place B--"

"Where is that?"

"Oh anywhere. I told you to think of two places, A and B."

"No, you told me to think of a place A, and I am still thinking of it,
because it is very hot there."

"Well, this is another place, where the pressure is simply frightful. When
the air rises at A the air from B rushes over to A to fill up the gap, and
that is what we call wind."

"I see."

"No, you don't. It isn't quite so simple as that. Now, the atoms of air
rushing from B to A don't go _straight_ there, but they travel in--in sort
of _circles_."

"Why do they do that?"

"Well, the fact is that these atoms are so keen to get over to A, where
there is plenty of room, that they jostle each other, and that makes them
go round and round. If they go round and round _against_ the clock, like
that, they are called cyclones, or depressions, or low-pressure systems. If
they go with the clock, like that, it is an anti-cyclone."

"Oh!"

"What do you mean--'Oh'?"

"What I said; but go on."

"Now suppose this air--"

"Which air?"

"The air from B. Suppose it is travelling in a cyclone--"

"But isn't a cyclone a low-pressure thingummy?"

"Yes."

"And didn't you say that B was a high-pressure place?"

"Yes."

"Then how does the air coming from B manage to be low-pressure stuff?"

"I see what you mean. There _is_ an explanation, but it would take too long
to hazard it now. Suppose the air is coming from B in an anti-cyclone, then
..."

"All right. I'll suppose that."

"... it rushes over to A and fills up the gap. There is more pressure at A
and the barometer goes up--"

"Is it fine then?"

"No, it rains. You see, the air from B is colder than the air at A was
before the air came from B."

"I _don't_ see."

"Well, obviously it _must_ be."

"How 'obviously'?"

"Well, the whole thing started with it being very hot at A, you remember,
so that the air rose. If it had been hotter still at B just then the air
would have risen at B instead, and it couldn't have rushed over to A.
There'd have been a frightful muddle."

"There is."

"Well, it's your own fault for interrupting. This air, then--"

"Which air is this?"

"The air from B. The air from B cools the air at A--"

"But I thought the air at A had risen."

"Not all of it. And that makes it rain."

"Why?"

"Oh, well, I can't go into that. It's something to do with conde