delivered the opinion.

If the tendency of parol evidence is to contradict, vary, or add to a written instrument, it cannot be received; if to explain and elucidate it, it may be received. Upon the face of this writing there is nothing doubtful or equivocal. It states a simple transaction, and imposes no obligation upon *266the defendant; but the object of the evidence is to shew, that when he made the contract, he entered into a stipulation, by which a duty was imposed upon him, for the breach of which, this action was probably brought. This is in effect, to prove by inferior evidence, that which purports, on the face of it, to be a memorial of the defendant’s contract, is in truth not so. Such evidence is inadmissible, according to all the authorities.