This Act shall not prejudice or affect the trial in manner heretofore in use of any act of piracy as defined by the law of nations, or affect or prejudice any law relating thereto; and where any act of piracy as defined by the law of nations is also any such offence as is declared by this Act to be within the jurisdiction of the Admiral, such offence may be tried in pursuance of this Act, or in pursuance of any other Act of Parliament, law, or custom relating thereto.