"The difference between [perjury and prevarication] consists in this, that perjury is a distinct assertion of that which can be proved not to be, or a denial of that which can be proved to be; whereas prevarication is the giving of contradictory or inconsistent evidence, which affects the credibility of the evidence, though neither the extent of the witness’s falsehood, nor the precise points in which he has departed from the truth be capable of ascertainment."