=64. Weight of Sash and Glass.=—In estimating the weight of window sash, in order that the size of the counterweights or of the spring counterbalance may be determined, the weight of the glass per square foot may be taken as follows: Plate glass, 3½ pounds; double-thick glass, 1½ pounds; and single-thick glass, 1 pound. To find the weight of the wooden sash, add together the height and the width of the sash, in feet, and multiply by 2.1 for 2¼-inch sash, by 1.67 for 1¾-inch sash, and by 1.33 for 1⅜-inch sash. The several sizes of sash given indicate the thickness of the sash frame. While these data for determining the weight of sash are not exact, they are sufficiently accurate to fix the size of the sash cords and pulleys and to estimate the weight required to counterbalance them. The best practice in counterbalance sashes, however, is to weigh the sash after it has been glazed; in this manner the exact weight and size of the counterweights required can be determined. The approximate weights of ordinary glazed sash are usually given in the catalogs of manufacturers of sash weights, pulleys, etc., and will be found convenient in determining the approximate weight of sash weights without making the calculations just described when estimating. =65. Sash Locks, or Fasts.=—There are many different makes of =sash locks= for double-hung sash in the market. In Fig. 74 are shown several of the older type of sash locks that have been used extensively. The lock shown at (_a_) is known as the _Champion_; that at (_b_), as the _Ives_, and that at (_c_), as the _Boston_. This type of sash lock, the construction of which is apparent from the illustration, has given satisfaction for a number of years. [Illustration: FIG. 74] [Illustration: FIG. 75] [Illustration: FIG. 76]