It is very far from my purpose to construct an antithesis between a purely scientific and a technical conception of the Problem of Mechanics. The conflict which undoubtedly existed some years ago, and at times was waged with bitterness, has come to a satisfactory conclusion; and a generous appreciation of the two different directions of investigation has taken the place of unseemly strife. From my personal knowledge of the Technical High School in Aachen, I at least can lay stress on the fact that I have been encouraged in the most ready manner by my technical colleagues of all departments.