The district court properly construed the written instruments which state the engagements of the parties. • The defendant desired to develop' the gas resources of his land and the plaintiff loaned him money to do so, agreeing to accept payment in gas. But the defendant agreed to prosecute development work by drilling additional wells whenever the requirements of the plaintiff demanded. The requirements of the plaintiff demanded additional wells, the defendant failed to drill them, gas was not produced and sold according to the contract, and the note ma*549tured under the provisions of the supplemental agreement.

It is not necessary to discuss the assignments of error seriatim.

Even if the third finding of fact be based upon an erroneous computation the validity of the conclusion of law is not affected. A carbon impression of a letter written on a typewriter, made with thé same stroke of the keys as the companion impression, is an original. It is not material which one is mailed and which one retained by the writer and either one may be offéred as primary evidence of the contents of the letter.

The judgment of the district court is affirmed.