"The falls of the Ohio are occasioned by an irregular ledge of rock stretching across the river. They are only perceptible at low water, the whole descent being but twenty-two feet, while the difference of level between the highest and lowest stages of the water is about sixty feet. When the river is full, they present, therefore, no serious obstruction to the navigation. To obviate the inconvenience, however, at low water, a canal, called the Louisville and Portland Canal, has been constructed round the falls, which is deserving of notice, as being, perhaps, the most important work of the kind ever undertaken. The cross section of the canal is 200 feet at the top of the bank, 50 feet at the bottom, and 42 feet deep, making its capacity about fifteen times greater than that contemplated for the Erie Canal after its enlargement is completed: its sides are sloping and paved with stone. The guard lock contains 21,775 perches of masonry, being equal to that of fifteen locks on the New York Canals; and three others contain 12,300 perches. This canal is capable of admitting steamboats of the largest class. It is scarcely two miles in length; but, considering the quantity of mason work, and the difficulty of excavating earth and rock from so great a depth, together with the contingencies attending its construction, from the fluctuations in the depth of the river, it is probably no over-statement when it is said, that the work in it is equal to that of seventy or seventy-five miles of an ordinary canal." LETTER XIII. NEW YORK--ASTOR LIBRARY.--COOPER INSTITUTE.--BIBLE HOUSE.--DR. RAE--DR. TYNG.--TARRYTOWN.--ALBANY.--SLEIGHING--FINAL RETURN TO BOSTON.--HALIFAX.--VOYAGE HOME.--CONCLUSION. Albany, Nov. 27th, 1858. My last letter was despatched to you on the 23rd inst.;--that evening we dined at Mr. Aspinwall's. He has a handsome house in New York, and a large picture gallery, and as we wished to see this by daylight, we called on him after breakfast on the following morning, and had an opportunity of examining the pictures, many of which are very good, especially some by early Dutch masters. Mr. Aspinwall afterwards took us to the Astor Library. This library was founded by the munificence of the late Mr. Astor, a very rich merchant, who bequeathed a large sum of money for the purpose. It is remarkably well arranged and pretty, and capable of containing about 300,000 volumes. Mr. Cogswell, the librarian, showed us some of the most valuable books. He was acquainted with Papa's name, as he had bought his book in London for the library, and appeared familiar with its contents. He said he valued it as filling up a gap in the financial history of America that was not supplied by any work in this country.