In a letter on the above subject from J. W. Truax, of Bickford, Vt., he states, that vibrations in some dams are caused by their imperfect construction, they generally not being sufficiently strong. This was the case with the dam in that place. Part of it was carried away by a freshet in February, 1857 ; this was repaired under Mr. Truax's superintendence, but not sufficiently, as he had not time to do so, owing to a second rise of the river. When after this, the water flowed over the dam twenty-four inches deep, vibrations were discovered for the first time, and they were so severe as to jar all the windows in the neighboring houses. When the question arose as to the cause of this, he replied that it we.s owing to the imperfect condition of the dam. In the month of June last, when the state of the river permitted, he examined the dam thoroughly, and found that his surmises were verified. He had the dam thoroughly repaired, and since then the vibrations have ceased. Vibrations in dams, therefore, are produced by more than one cause ; but this still confirms our former opinion, that they can be prevented by the proper construction of dams.