Charles Evans Hughes, Wilson v. United States (221 U.S. 361…
“ Long, 9 East, p. 484: 'The right to resort to means competent to compel the production of written, as well as oral, testimony, seems essential to the very existence and constitution of a court of common law, which receives and acts upon both descriptions of evidence, and could not possibly proceed with due effect without them.' Where the subpoena duces tecum contains the usual ad testificandum clause, still it is not necessary for the party requiring the production to have the person producing the documents sworn as a witness. They may be proved by others. 3 Wigmore, Ev. ”
