The following Letters on Quarantine, Yellow Fever, and Cholera Asiatica, were hastily written at the request of a Committee of each branch Of the Legislature of the State. Originally printed as Pub lic Documents at Albany, with no supervision of the proofs by myself, unavoidable errors were the result. Under such circumstances, I have been induced to leisurely revise them, and to add a few notes, in order that they may be published in a more perfect form.