We should be sorry to weaken the rule of law, which provides that parol evidence shall not be admitted to contradict, vary or alter the terms of a written contract; but the stability dnd value of the rule itself depends on its being applied with care and just discrimination. An undertaking to pay a certain rate of freight, “ with primage and average accustomed,” in our opinion means such primage and average as the custom of that trade warrants and requires; if it requires none, then none is payable by force of the obligation. The parties appeal to such custom, as the test and measure of the liability of the one to the other. The contract itself calls for evidence aliunde to give it effect; and if the fact to be proved *595is one which can be proved only by parol evidence, parol evidence is necessary and competent. A contract for the present sale of merchandise—say wheat—at a rate to be determined by the average price of wheat in Boston market at a specified future day, would necessarily call for and admit parol evidence to prove what that price was, after the day has passed; because that alone can give effect to the terms of the contract; but it in no sense impugns it, or alters its terms or legal effect. The same rule applies to deeds and other contracts, however solemn and formal. The actual position of monuments, boundaries and other local objects referred to, may be proved by the appropriate evidence, which is usually parol.

The plaintiffs, as consignees, had a right to receive the goods, only on condition of paying the ship-owner, or his agent the master, the stipulated price for their carriage, specified in the bill of lading, but no more. The ship-master had a lien on them to that extent, and no more. By demanding and receiving the goods under these circumstances, by which the lien was discharged, the law implies an obligation on their part to pay such price, though there were no express promise. The true price for carriage was the measure of their liability. If the defendant demanded more, and the plaintiffs paid more, through mistake of fact, they have a right to recover it as money had and received to their use.

Whether the evidence offered would have proved the custom relied on, showing that primage, by the custom of that trade, was not payable, we are not called on to judge; but we are of opinion that the evidence ought to have been received and submitted to the jury.

Exceptions sustained.