In anticipation of some points to be explained only if these phenomena are understood, I may simply remark here that, obviously, if some parasite attacks the growing lips of the "callus" as it is trying to cover up the wound, or if the cambium is injured below, the pathological disturbances thus introduced will modify the result: the importance of this will appear when we come to examine certain disturbances which depend upon the attacks of fungi which settle on these wounds before they are properly healed over. In concluding this brief sketch of a large subject, it may be noted that, generally speaking, what has been stated of branches, etc., is also true of roots; and it is easy to see how the nibbling or gnawing of small animals, the pecking of birds, abrasions, and numerous other things, are so many causes of such wounds in the forest. (_To be continued._) SIBLEY COLLEGE LECTURES.--1887-88. BY THE CORNELL UNIVERSITY NON-RESIDENT LECTURERS IN MECHANICAL ENGINEERING. III.--THE EVOLUTION OF THE MODERN MILL.[3] By C. J. H. WOODBURY, Boston, Mass. BELT TOWERS. The distribution of power has not always received the judicious treatment which its importance deserves. There are but few references to this question in the books on the subject, and these treat of methods that are not in accordance with the application of the art in its present state. [3] Continued from SUPPLEMENT, No. 647, page 10331. The lecture was illustrated by about fifty views on the screen, which cannot be reproduced here, showing photographs of mills and mechanical drawings of the methods of construction alluded to in the lecture. The early form of the distribution of power consisted in placing a vertical shaft extending through the whole mill and distributing the power at each story by means of beveled gears, generally of skew-beveled form. The mechanical defects of such a method of distributing power, with regard to protection, repairs, and necessary care, are readily apparent, and there have also been many severe accidents caused by the breaking of teeth in these gears. The present method of distributing power in this country is entirely by lines of belts extending up through what is known as a belt tower, which constitutes an element of great fire hazard to a mill. In some cases the belts are carried from story to story, covered by a casing of wood, and in other instances the tower forms a flue which may be the means of the rapid spread of fire throughout the building.