[XXIX]
CONSTRUCTION
Fig. 153
The study of construction includes many items such as strength, proportion, joints, etc. If we look at the roof timbers shown in outline at a ([Fig. 153]), the interesting parts of the construction are the three spaces enclosed by circles. The straight lines between these circles do not interest us very much, but the parts enclosed do. Immediately the question arises, how are the timbers fastened at these places? In other words, what kind of joint is used? The joint then is the critical part, or we might say the cream of the construction.
A very large number of joints are in use, but many of them are rare. Our grandfathers, who built their houses and barns from oak timbers hewn out with the axe, commonly used the mortise and tenon, fastened with a generous hard wood pin, and many of them are still standing after a century or two of hard usage. The fact that the beams were rough hewn, instead of sawed, did not in any way affect their strength, because they made good, strong joints.
Some of the more common joints are shown in the accompanying illustrations, and may be used for reference.
No. 1. A butt joint in which the two pieces are fastened together, end to end, by means of glue and dowels. It should be used only in cases where there is little strain in the direction of the two pieces.
No. 2. A dowel joint joining two pieces at right angles. One form of it is shown at No. 3 applied to the leg of a table.