"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—"