 When Washington Irving was appointed minister to Spain, he was anxious that his friend Cogswell should accompany him as secretary of legation, and accordingly wrote to Washington, requesting his appointment.“He is,” said Irving, “a gentleman with whom I am on terms of confidential intimacy, and I know no one who, by his various acquirements, his prompt sagacity, his knowledge of the world, his habits of business, and his obliging disposition, is so calculated to give me that counsel, aid, and companionship, so important in Madrid, where a stranger is more isolated than in any other capital of Europe.”

Cogswell received the appointment, and would probably have accepted it, but, Astor finding that he was likely to lose his invaluable services, made him superintendent of the new library.After the rich merchant's death, in 1848, Cogswell traveled to Europe to purchase books.