The Wealth of Nations by Part 4 Chapter 4 Page 13

They only prevent its being excluded altogether by those duties. The carrying trade, though it deserves no preference, ought not to be precluded, but to be left free like all other trades. It is a necessary resource for those capitals which cannot find employment either in the agriculture or in the manufactures of the country, either in its home trade or in its foreign trade of consumption.

The revenue of the customs, instead of suffering, profits from such drawbacks by that part of the duty which is retained. If the whole duties had been retained, the foreign goods upon which they are paid could seldom have been exported, nor consequently imported, for want of a market. The duties, therefore, of which a part is retained would never have been paid.

These reasons seem sufficiently to