"Let x and y be respectively the base and the height of a right-angled triangle (Fig. 4), of which the slope... is fixed at 30º. If we suppose this triangle to expand and yet keep its angles the same as at first, then, when the base grows so as to become x + dx the height becomes y + dy. Here, increasing x results in an increase of y. The little triangle, the height of which is dy and the base of which is dx is similar to the original triangle; and it is obvious that the value of the ratio dy/dx is the same as that of the ratio y/x."