Peterboro, August 15, 1848. : J.K. Ingalls, editor of the Landmark, New-York: My Dear Sir, --I have your letter. I had learned, before receiving it, of the report in Buffalo, during the late convention there, that I was in favor of the nomination of Mr. Van Buren, and would vote for him. ... But it is not true, that I shall vote for Mr. Van Buren. I can vote for no man for president of the United States, who is not an abolitionist ..