It is perhaps a rather startling question to put to the learned and accomplished members of such an Association as this—What is sound ? All your lives long you have doubtless held very firm and clear convictions on the subject of Sound; and have trustingly accepted the theory which has obtained, one may almost say, since the time of Pythagoras, and which has, in our day, been fostered by all the great acousticians, and dogmatically taught by Professors Tyndall, in England, Helmholtz, in Germany, and Mayer, in America.