The Wealth of Nations by Part 4 Chapter 5 Page 115

No bounty had ever been given before for the exportation of this grain, no more than for that of pease or beans.

By the same law, too, the exportation of wheat is prohibited so soon as the price rises to forty-four shillings the quarter; that of rye so soon as it rises to twenty-eight shillings; that of barley so soon as it rises to twenty-two shillings; and that of oats so soon as they rise to fourteen shillings. Those several prices seem all of them a good deal too low, and there seems to be an impropriety, besides, in prohibiting exportation altogether at those precise prices at which that bounty, which was given in order to force it, is withdrawn. The bounty ought certainly either to have been withdrawn at a much lower price, or exportation ought to have been allowed at a much higher.