Whenever any person has been examined as a witness, either for the Commonwealth or for the defense, in any criminal proceeding conducted in or before a court of record, and the defendant has been present and has had an opportunity to examine or cross-examine, if such witness afterwards dies, or is out of the jurisdiction so that he cannot be effectively served with a subpoena, or if he cannot be found, or if he becomes incompetent to testify for any legally sufficient reason properly proven, notes of his examination shall be competent evidence upon a subsequent trial of the same criminal issue. For the purpose of contradicting a witness the testimony given by him in another or in a former proceeding may be orally proved.