Messrs. Editors.—In the last number of your paper I notica an article on the " Inter-oceanic Canal to the Pacific," which, so far as the report of Lieut. Craven is concerned, is perfectly accurate, but is, I think, calculated to mislead those who are not acquainted with the previous history of the project; and as the subject is one of great and universal interest, it is important that no undue prejudice be raised against it. The proposed route was originally explored by W Kennish, Esq., C.E., whose plans and estimates were published on his return, and submitted to the consideration of eminent engineers, both of this country and in Europe. Their opinion as to the practicability of constructing a canal, without locks, sufficientl} capacious for the passage of the largest ves-sels from ocean to ocean, was unanimously favorable, provided the data furnished by Mr Kennish should be found correct. The expedition under Lieut. C. was theref ore sent, noi to survey any new route, nor to make furthei explorations, but merely to verify the statements of Mr. Kennish. He has not contradicted a single one of these statements so far and his hasty condemnation of the project is, therefore, wholly without reason, for all th( difficulties he urges against it were met anc estimated for, in the report of the origina survey. In this state of the case it is impos-sible to pronounce judgment until the repori of Eteut. Michler, Topographical Engineer o the late expedition, shall have appeared, whei the question will be settled by the scientifi( world. These facts should be made known, in jus-tice to the promoters of an enterprise of which if successfully completed, the whole world may well be proud. Yours, James A. Rockwell. New York, June, 1858.