A correspondent in Vermont writes to us stating the following case : " There is in this place a new dam 65 feet long, nearly level on the top, with a smooth edge ; and when the water flows over it in a thin and unbroken sheet, there is always a vibration of the water from the bottom to the top of the dam, and back in the pond for three or four feet ; it also causes the windows of houses about the falls to jar and rattle. What is the cause of it ?" The vibration is caused by thc water falling on to the ground from the dam or into the lower water ; and it will, of course, affect all bodies in its neighborhood that are capable of moving. However evenly the water may pass over the dam, it will be uneven in its descent, and so cause the snbstance on which it falls to v'brate, and by concnssioll affect the dam itself.Eds.