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Brown v. Maryland

An impost, or duty on imports is a custom or a tax levied on articles brought into a country, and is most usually secured before the importer is allowed to exercise his rights of ownership over them, because evasions of the law can be prevented more certainly by executing it while the articles are in its custody. It would not, however, be less an impost or duty on the articles, if it were to be levied on them after they were landed.... What, then, are "imports?" The lexicons inform us they are "things imported." If we appeal to usage for the meaning of the word, we shall receive the same answer. They are the articles themselves which are brought into the country. "A duty on imports," then, is not merely a duty on the act of importation, but is a duty on the thing imported. It is not, taken in its literal sense, confined to a duty levied while the article is entering the country, but extends to a duty levied after it has entered the country.