“Though this motion for a new trial is an application to the discretion of the Court, it must be remembered that the discretion to be exercised on such an occasion is not a wild but a sound discretion, and to be confined within those limits within which an honest man, competent to discharge the duties of his office, ought to confine himself. And that discretion will be best exercised by not deviating from the rules laid down by our predecessors; for the practice of the Court forms the law of the Court.”