No rule of evidence is more uniformly observed, in the administration of justice, than that which precludes the admission of oral testimony to control, vary or explain the terms or the construction of a written contract. The rule is founded on a wise policy and a just principle, to prevent the perversion of justice by the falsehood, mistake or mis-recollection of witnesses. To this, as to all other general rules, there are exceptions : As in cases of latent ambiguities, or where words are used in contracts relating to a particular trade or business, which are not generally understood, or which may have acquired a peculiar meaning by local usage. None of these exceptions are applicable to the present case. The parol evidence offered at the trial was to prove an agreement, made by the parties at the time of signing the contract, as to the time of payment for the wood sold. This evidence, if admitted, would have materially varied the construction of the written contract, though it would not have expressly contradicted its terms. No time of payment for the wood was named in the contract. It was therefore, by a well established rule of law, payable on demand.

In Atwood v. Cobb, 16 Pick. 231, the chief justice expressed an opinion, that in an action on a contract in writing, which does not express the time when it is to be performed, it is to be performed within a reasonable time; and that a simultaneous, *100express verbal agreement, that it shall be performed on a particular day, is not admissible in evidence. This opinion we consider well founded, with the' qualification that when the act to be done is the payment of money, the payment is to be made on demand; this being the well settled legal, construction of such a promise. In other cases, the nature of the act to be done, and other facts and circumstances which may be proved, are to be taken into consideration, as bearing on the question of reasonable time. But a simultaneous express agreement as to such a question is not admissible in evidence.

We are therefore of opinion that the oral testimony, offered by the defendants’ counsel,~tvas rightly rejected, and that the ,‘nstruction was correct.

Exceptions overruled.