Along the axis of such columns a hole of about one and one-half inches in diameter is bored, and near each end a couple of transverse holes, generally half an inch in diameter, furnish means of ventilating the inside of the column for the prevention of dry rot and also checking, due to contraction and seasoning. There are several methods of laying the floor plank upon these beams, which are placed from eight to ten feet apart, according to the dimensions of the machinery to be placed in the mill. The first floor of three-inch plank, planed on one side and grooved on both edges, is laid planed side down, and the hardwood splines are inserted into the grooves before the planks are pressed up and spiked to the beams. An agreeable finish is sometimes arranged underneath by plowing a rabbet in each of the corners, and inserting a bead in the groove thus formed, which is secured by nails driven diagonally into the plank on one side only, because if the nails were driven into both sides, the bead would be split by the contraction of the plank. These planks should be cut to sufficient length to cover two bays of the mill; and their transverse resistance is that of a beam fixed at one end and supported at the other, or one and three-fifths as much as a plank of the same size but half the length would support; but it should be remembered in this connection that, if evenly distributed on the floor, five-eighths of the load would be carried by every alternate beam unless the planks are so laid to break joints at convenient intervals of about three feet. The top flooring is generally laid directly upon the floor plank, with one or two thicknesses of roofing paper interposed; but the preferable method, which deadens the sound and vibration, and also greatly increases the fire-resisting qualities of the structure, is to lay a coat of mortar on the floor plank, preserving the uniform thickness by means of furring placed about sixteen inches apart, and then to lay the upper floor upon this. For these upper floors hardwood plank, one and one-fourth inches thick, and not over four inches wide, is used. The black birch is considered by many to possess the greater resistance to wear; and Southern pine is ranked next, although the latter wood gives trouble by stringing, especially when trucks are rolled over it. White maple forms an excellent top floor, although not so hard as others, especially where the floor is likely to be exposed to water, as in paper mills and bleacheries. ROOFS. Benjamin Franklin once said that next to a good foundation a good roof was the most important feature of a building. Although the constructive features of mill roofs are well defined, yet with regard to roof covering there is a wide diversity of experience and opinion.