“Immemorial usage preserves the memory of positive law, long after all traces of the occasion, reason, authority and time of its introduction, are lost; and in a case so odious as the condition of slaves, must be taken strictly; (tracing the subject to natural principles, the claim of slavery never can be supported.) The power claimed by this return never was in use here. We cannot say the cause set forth by this return is allowed or approved of by the laws of this kingdom; and therefore the man must be discharged.”