form is
not available for wide spans, and is shown to illustrate how the
development was made into the succeeding types.

[Illustration: _Fig. 290. Braced Collar Beam._]

THE RIB AND COLLAR TRUSS, Fig. 291, is the first important structural
arrangement which permitted the architect to give full sway to
embellishment. The inwardly-projecting members (A, A) are called _Hammer
Beams_. They were devised as a substitute for the thick walls used in
the Braced Collar Beam Truss, and small brackets (B, B) were placed
beneath as supports.

[Illustration: _Fig. 291. Rib and Collar Truss._]

The short tie beam (C), near the apex, serves as the member to receive
the thrust and stress of the curved ribs (D, D). It forms a most
graceful type of roof, and is capable of the most exquisite
ornamentation, but it is used for the high pitched roofs only.

[Illustration: _Fig. 290-1/2. Hammer Beam Truss._]

The acme of all constructions, in which strength, beauty, and capacity
for ornamentation are blended, is the _Hammer Beam Truss_. Here the
hammer beam projects inwardly farther than in the preceding figure, and
has a deeper bracket (B), and this also extends down the pendant post
(C) a greater distance.

The curved supporting arch (D), on each side, is not ribbed, as in the
Rib and Collar Truss, but instead, is provided with openwork (not shown
herein), together with beadings and moldings, and other ornamental
characteristics, and some of the most beautiful architectural forms in
existence are in this type of roof.

What are called Flying Buttresses (E) are sometimes used in connection
with the Hammer Beam Truss, which, with heavy roofs and wide spans, is
found to be absolutely necessary.

CHAPTER XX

ON THE CONSTRUCTION OF JOINTS

In uniting two or more elements, some particular type of joint is
necessary. In framing timbers, in making braces, in roof construction
and supports, in floor beams, and in numerous other places, where
strength is required, the workman should have at his command a knowledge
of the most serviceable methods.

Illustrations can most forcibly convey the different types; but the
sizes must be determined by the character of the material you are
working with. Our aim is to give the idea involved, and the name by
which each is known.

[Illustration: _Fig. 292. Bridle Joints._]

Reference has been made in Chapter X, to certain forms of scarfing and
lapping pieces. This chapter has to do with