 In 1678 he made a spirited but ineffectual attempt to secure the conviction of Lord Cornwallis for the brutal murder of a boy in St. James's Park.The severe censure which Lord Campbell passed upon him for his conduct of this case is based upon an entire misapprehension of the facts.In the debate on Lord Danby's impeachment (December 1678) Maynard showed a regrettable disposition to strain the Treason Act 1351 (25 Edward III) to his disadvantage, maintaining that its scope might be enlarged by retrospective legislation, which caused Swift to denounce him, in a note to Burnet's "Own Time", as 'a knave or a fool for all his law.'