"What _is_ pressure? Who is pressing what?"

"Pressure is what the barometer tells you--not the thermometer; you must
keep the thermometer out of this. Suppose it is very hot in London--"

"Don't be ridiculous."

"Well, suppose it is very hot at a place A--"

"I thought we were keeping the thermometer out of this."

"It comes in indirectly. But don't keep interrupting. If it is very hot at
the place A, the air at A rises. You see?"

"No."

"Obviously it does. If you light a candle--"

"Yes, yes, I do see that. Don't begin about candles."

"Well, the result of that is that there is less _pressure_ at A. In other
words, 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."