Braley, J.
Upon omitting other provisions relating to the price and the depth that might be necessary to obtain the stated amount of supply, but not material to the principal question involved, the plaintiffs undertook by their contract “ to procure water in the earth above the bed rock” on the defendant’s premises by driving in the boiler room of the factory a pipe two and one half inches in diameter, but if a sufficient amount was not obtained then to “ drill a well not less than six inches in diameter in the bed rock . . . until twenty-five gallons of water per minute is obtained.” They finally procured this volume, but the water was very salt, and generally unsuitable for use in the defendant’s business, and, it being common knowledge that water in its natural state either may be fresh or salt in quality, the defendant contends that as the contract failed to designate the ordinary sense in which the parties must have used the term, oral evidence was admissible to explain and remove the uncertainty. Unless fraud or mistake are shown, where the parties have put their contract in writing, there is a conclusive legal presumption, that it contains the entire agreement in which all previous verbal negotiations concerning the subject matter have been merged. Violette v. Rice, 173 Mass. 82. DeFriest v. Bradley, 192 Mass. 346. But if any of the essential terms of the contract when applied to the transaction concerning which the parties dealt becomes ambiguous, oral evidence is relevant and admissible, not to construct a new agreement, but to ascertain what they understood by the one already made. Stoops v. Smith, 100 Mass. 63. Hebb v. Welsh, 185 Mass. 335. Buffington v. McNally, 192 Mass. 198. These far miliar principles are undisputed, but the difficulty arises in their application, especially where the agreements are drawn by the parties or their business agents, who fail to use clear and exact language to express their mutual understanding. It was the plaintiffs’ interpretation, that they had fully performed their undertaking if the amount of water was procured, even if it was wholly unsuitable either because of saltness or impurity. The *200defendant then offered evidence, that during the preliminary negotiations the agent of the plaintiffs was informed and knew that its object in driving the well was to procure a supply of water for drinking and other uses in its manufactory, for either of which salt water could not be used, and would be unserviceable. This offer also included conversations between their respective agents in which the location of the factory, and the accessibility to salt water, which, if desired, could be obtained at a small expense, was spoken of, as well as a statement by the plaintiffs’ agent, that they would furnish water of a quality equally good, when compared with water obtained for another customer, the quality of which was known to the defendant’s agent, who thereupon said, that if this were done the defendant would be satisfied. The testimony was excluded, upon the ground that conversations previous to the making of the contract must be held to have been merged, and oral evidence in explanation of the use and meaning of the word “ water ” therefore was inadmissible. But, while this rule is undoubtedly correct, and should be inflexibly applied where no ambiguity or uncertainty appears, when the parties by the language they have employed leave their meaning obscure and uncertain when applied to the subject matter, then .the expressions and general tenor of speech used in the previous negotiations, even if coming as they usually must from one or the other of the parties themselves, are admissible to show the conditions existing at the time the transaction was under consideration. See for illustrations Bradford v. Manly, 13 Mass. 139; Hogins v. Plympton, 11 Pick. 97; Stoops v. Smith, ubi supra ; Miller v. Stevens, 100 Mass. 519; Pike v. Fay, 101 Mass. 134; Sweet v. Shumway, 102 Mass. 365; Keller v. Webb, 125 Mass. 88; New England Dressed Meat & Wool Co. v. Standard Worsted Co. 165 Mass. 328; Boak Fish Co. v. Manchester Assurance Co. 84 Minn. 419. Their definition when thus ascertained furnishes the best interpretation of their contract, the construction of which still remains as a question of law for the court. Atwood v. Cobb, 16 Pick. 227, 232. Stoops v. Smith, ubi supra. Bassett v. Rogers, 162 Mass. 47. Lynn Safe Deposit & Trust Co. v. Andrews, 180 Mass. 527, 533. Callender, McAuslan & Troup Co. v. Flint, 187 Mass. 104. Buffington v. McNally, ubi supra. American Malt*201ing Co. v. Souther Brewing Co. 194 Mass. 89. United States v. Peek, 102 U. S. 64. Smith v. Faulkner, 12 Gray, 251, 255.
The evidence, therefore, was excluded wrongly, and as there must be a new trial at which the other questions raised by the exceptions may become immaterial, or be presented in another form, we do not consider them.
Exceptions sustained.