A COMPETENT WITNESS IN THE JOHN BROWN INVESTIGATION. – An interesting trial took place at Syracuse recently, of parties connected with the system of running off slaves by the underground railroad, about which Rev. Samuel J. May seemed to know a great deal. Mr. May, we believe, is an agent of the underground institution, and he might be able to give some important information relative to the Harper’s Ferry outrage. We suggest to the Senate Committee now investigating this affair at Washington to send for the Rev. Samuel J. May.