54. When an opening is too wide to be spanned with one or two pieces of timber, it may sometimes be effected by a combination of pieces which bear some resemblance to an arch in masonry; the principles of stability are not, however, exactly the same. In carpentry stability is gained chiefly by the resistance of the material to stretching and compression, and the mode of connection. In a stone arch it is attained by a proper disposition of the parts, so that each may be retained in its place by pressure alone, but in carpentry to lose the advantage of connection is to lose that in which the excellence of the carpenter's art chiefly consists.

Fig. 32.