Let me first speak of witnesses and their testimony. It is sometimes supposed that the desire to be veracious is the only quality essential to form a trustworthy witness--and an essential quality it is beyond all doubt--but it is possessed by many who are nevertheless very unsafe guides to truth. In the first place, this general desire for truth in a mind not carefully regulated, is apt to give way, oftentimes unconsciously, to impressions which overpower habitual veracity. It may be laid down as a general rule that witnesses are partisans, and that, often without knowing it, their evidence takes a color from the feeling of partisanship, which gives it all the injurious effects of willful falsehood--nay, it is frequently more pernicious. The witness who knowingly perverts the truth, often betrays his mendicity by his voice, his countenance, or his choice of words; while the unconscious perverter gives his testimony with all the force of sincerity. Let the witness who intends to give evidence worthy of confidence, be on his guard against the temptations to become a partisan. Witnesses ought to avoid consorting together on the eve of a trial; still more, discussing the matters in dispute, and comparing their intended statements. Musicians have observed that if two instruments, not in exact accordance, are played together, they have a tendency to run into harmony. Witnesses are precisely such instruments, and act on each other in like manner. So much with regard to the moral tone of the witness; but the difficulties which I have pointed out may be surmounted, and yet leave his evidence a very distorted narrative of the real facts. Consideration must be given to the intellectual requirements of a witness. It was the just remark of Dr. Johnson that complaints of the memory were often very unjust toward that faculty which was reproached with not retaining what had never been confided to its care. The defect is not a failure of memory, but a lack of observation; the ideas have not run out of the mind--they never went into it. This is a deficiency, which cannot be dealt with in any special relation to the subject in hand; it can only be corrected by cultivating a general habit of observation, which, considering that the dearest interests of others may be imperiled by errors arising out of the neglect to observe accurately, must be looked upon in the light of a duty. A still greater defect is the absence of the power of distinguishing fact and inference. Nothing but a long experience in Courts of Justice, can give a notion of the extent to which testimony is adulterated by this defect. It is often exemplified in the depositions of witnesses, or rather in the comparison between the depositions which, as your readers know, are taken in writing before the committing magistrate, and the evidence given on the trial.