A. Its principal use is to hold the crust up, and prevent it from sinking, when the cooked fruit gives away under it.
Q. Does not the cup prevent the fruit of the pie from boiling over?
A. No, by no means; it would rather tend to make it boil over, than otherwise.
Q. Why would the cup tend rather to make the fruit boil over?
A. As soon as the pie is put into the oven, the air in the cup will begin to expand, and drive every particle of juice from under it; the pie dish, therefore, will have a cup-full less room to hold its fruit, than if the cup were taken out.
Q. If the juice is driven out of the cup, why is the cup always full of juice, when the pie is cut up?
A. Immediately the pie is drawn, the air in the cup begins to condense again, and occupy a smaller space; in consequence of which, there is no longer enough air to fill the cup, and so juice rushes in to fill up the deficiency.
Q. Why does juice rush into the cup, because the cup is not full of air?
A. As the external air presses upon the surface of the juice, it rushes into the cup unobstructed; as mercury rises through the tube of a barometer through similar pressure.