§ 1. We have now, in order, examined the means of raising walls and sustaining roofs, and we have finally to consider the structure of the necessary apertures in the wall veil, the door and window, respecting which there are three main points to be considered.

1. (1.) The form of the aperture, i.e., its outline, its size, and the forms of its sides.

2. (2.) The filling of the aperture, i.e., valves and glass, and their holdings.

3. (3.) The protection of the aperture, and its appliances, i.e., canopies, porches, and balconies. We shall examine these in succession.

§ 2. (1.) The form of the aperture: and first of doors. We will, for the present, leave out of the question doors and gates in unroofed walls, the forms of these being very arbitrary, and confine ourselves to the consideration of doors of entrance into roofed buildings. Such doors will, for the most part, be at, or near, the base of the building; except when raised for purposes of defence, as in the old Scotch border towers, and our own Martello towers, or, as in Switzerland, to permit access in deep snow, or when stairs are carried up outside the house for convenience or magnificence. But in most cases, whether high or low, a door may be assumed to be considerably lower than the apartments or buildings into which it gives admission, and therefore to have some height of wall above it, whose weight must be carried by the heading of the door.