Arrest of a supposed spy.
--The report prevailed in this city last evening that a son of the law partner of Louis Mackenzie, whose office is in Alexandria, the last-named member of which firm is well known in this city as a violent Union man, and President of the Loudoun & Hampshire Railroad, was yesterday taken a prisoner by several South Carolina volunteers. On examining his person a map of Harper's Ferry was found adroitly concealed in the collar of his coat, together with a manuscript embodying the writer's ideas in regard to the effective force and disposition of the forces at that point, and the most probable way in which an attack could be made by the Federal troops with the likelihood of success. With this evidence upon him he is said to have been hung as spy.--We give the report for what it is worth. It came from a gentleman who conversed with another who saw the body hanging and gave him the facts.
