Where the windows of the basement, as shown at m, are brought down close to the pavement, it is absolutely necessary that the pavement be sloped away from these windows with considerable pitch, not less-than 1 inch in 1 foot, as otherwise the water is likely to lay against the window sill or run under it, causing it to rapidly decay, the capillary attraction of the window frame drawing up the water. [Illustration: FIG. 10] =29. Window Openings.=—Referring to Fig. 9, which shows the face view of a bay of the wall illustrated in section in Fig. 7, the details of the several window openings in the walls may be studied. The basement windows are independent frames with double-hung sash, a rowlock brick arch supporting the brickwork over the window head. In the practice and design of window heads for mill buildings, it is usual to make the radius of the window head equal to the width of the reveal. In this instance, the distance across the opening is 4 feet 3 inches, and the radius of the arched head is the same dimension. The windows throughout the balance of the building are twin windows, double hung, and the construction of the window frame and sash is shown in the drawing. This frame is what is known as a =reveal frame=, and is built in as the brickwork progresses. Sometimes the frame is slipped in from the back, as shown in Fig. 10, and when this is the case the work can be carried along without waiting for the window frames. [Illustration: FIG. 11] As distinguishable from the reveal frame, there is the =plank-frame construction=, which is not built into the brickwork, but is built up as shown in Fig. 11. When it is desirable to have the central mullion _a_, Fig. 9, as narrow as possible, the box construction indicated on the drawing is done away with and the window is hung by means of overhead pulleys, the weights operating in the boxes at the sides. STANDARD SLOW-BURNING CONSTRUCTION =30.= A type of factory construction more usual than that previously described is illustrated in Fig. 12. In this illustration, it will be noticed that the main girders bear on wall pilasters, and the spandrel filling between the pilasters is kept as thin as possible. The usual reveal window frame is used, as shown at _a_, and the soffit of the arch over the window openings is checked at the head of the opening to provide a wind and water stop as at _b_. In this construction, which is probably the best, though it does not possess the advantage of giving the maximum amount of window space, and, consequently, light in the building, a rowlock or bonded brick arch is used over the window frames. By means of this construction, either the window frame may be built in place, or the windows may be slipped in from the back against a rabbet formed in the brickwork. The arch over the window head is indicated at _c_. [Illustration: FIG. 12] [Illustration: FIG. 13]