It lias already been mentioned that window frames are of two kinds, i.e. solid and hollow, the latter being known also as boxed or cased. These will now he described in turn.
If the sash is to be fixed, the rebate should be on the outside of the frame, as in Fig. 543, for then the pressure of the wind tends to tighten the joint between them; but if the sash is to be movable, the rebate may be either outside, as in Fig. 543, or inside, according to the way the sash is hung.
It is a very common practice to fix solid frames with the rebate inside, as it is often convenient for the sash to open inwards; but it is an advantage to have the rebate outside if possible, for in that case any water which finds its way in between the sides of the frame and the sash is stopped by the projection of the rebate, against which the sash shuts, whereas when the rebate is inside, any water penetrating at the sides is conducted downwards until it reaches the sill and trickles over it into the room.
The sill1 (os, Fig. 543) is generally made of hard wood, such as oak or pitch pine, as it is much exposed; its upper surface is bevelled to fit the lower rail of the sash, '•' weathered " to throw off the water, and frecpuently throated, as in Fig. 545, to prevent the water from being blown along it; occasionally it is double throated, as in Fig. 540.
In order to prevent the wet from working in underneath the oak sill, a metal tongue or "water bar" (wb) is inserted between it and the stone sill, as shown in Fig. 543, or a step is made in the upper surface of the latter, as in Fig. 540. This last arrangement is unusual and expensive.
Figs. 542, 543 show the external elevation and cross section of a small fixed sash in a solid frame. The plan below (Fig. 541) is on double the scale of the other figures.