I transmit, herewith, a copy of a letter from Mr. John Mills, of Missouri, to the Postmaster General; and another, of a letter from the latter gentleman to me, relative to the alleged abduction of Mr. George Abby, a citizen of the United States, by Indians of the Pawnee tribe, roaming within the confines of Mexico. As, by the 33d article of the treaty, provision is made for the occurrence of a case like this, you are instructed to require the aid of the Mexican Government towards effecting the recovery of Mr. Abby.