A correspondent of the London Garden writes that of all the grand objects he saw in America during his visit to the great Centennial Exhibition in Philadelphia, those which impressed him the most permanently, were Niagara Falls ; a certain sunset effect on a river with floating masses of dazzling white snow on it when the water seemed the color of the brilliantly red sky which was reflected in it; and the huge trees of Magnolias conspicua with their thousands of large white blossoms, which are not uncommon about Philadelphia.