Texas Supreme Court July 1, 1863 (Reese v. Medlock)

Moore, J.

It is a well settled general principle, that, when an agency is created and conferred by a written instrument, the-nature and extent of the authority given by it must be ascertained from the instrument itself, and can not be enlarged by parol evidence of the usage of other agents in like cases, for that would be to contradict or vary the- terms of the written instrument.