This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The construction of the wooden portion of forcing-frames is illustrated in Fig. 617, and described below; the fixing of the glass portion will be found under Glazing. A convenient length for the frame is 6 ft., and the width may be either 4 ft. for single or 8 ft. for double size. It is an advantage to have a frame that will take to pieces, and the one shown is designed with that object. The sides a, foot b, and head c are of l 1/4-in. deal. The top edge of the sides a is cut with a slope so as to allow the glass lid to be at an angle of about 22° 30'; therefore if the foot b is 1 ft. high, the head c on a frame 6 ft. long will be over 3 ft. high. The ends of the foot and head boards b c are halved into the ends of the sides b, so as to make a good joint. Into the ends of b c, staples are driven, and notches are cut out of a to admit them; small bars or wedges are thrust into the projecting loops of these staples in order to secure the sides and ends together in place. Halved into the top edge of the sides a are 2 strips d, measuring about 2 in. by 1 in. These are firmly screwed to the sides and constitute guards for the sliding sash e, to prevent it slipping sideways off the frame.
In a double frame there must be a central bar, 3 in. by 2 in., run from the head to the foot of the frame to carry the inner edges of the sashes, and this should have a strip 3/4 in. wide placed edgewise down the middle to separate the 2 sashes. On the top edges of the sides a, and similarly in the upper surface of the central bar, little channels should be grooved out to carry away any water that may find its way under the edge of the sashes. The sashes themselves are made of 2-in. by 1-in. quartering, dovetailed at the corners, with small bars for carrying the glass, as described on p. 348.