used in longer structures. Both of these forms are equally
well adapted for small bridges or for roof supports.

THE VERTICAL UPRIGHT TRUSS.--This form of truss naturally develops into
a type of wooden bridge known all over the country, as its framing is
simple, and calculations as to its capacity to sustain loads may readily
be made. Figs. 237, 238 and 239 illustrate these forms.

[Illustration: _Fig. 236._]

[Illustration: _Fig. 237._]

THE WARREN GIRDER.--Out of this simple truss grew the Warren girder, a
type of bridge particularly adapted for iron and steel construction.

This is the simplest form for metal bridge truss, or girder. It is now
also largely used in steel buildings and for other work requiring
strength with small weight.

[Illustration: _Fig. 238._]

[Illustration: _Fig. 239._]

[Illustration: _Fig. 240._]

THE BOWSTRING GIRDER.--Only one other form of bridge truss need be
mentioned here, and that is the _bowstring_ shown in Fig. 240.

In this type the bow receives the entire compression thrust, and the
chords act merely as suspending members.

FUNDAMENTAL TRUSS FORM.--In every form of truss, whether for building or
for bridge work, the principles of the famous A-truss must be employed
in some form or other; and the boy who is experimentally inclined will
readily evolve means to determine what degree of strength the upper and
the lower members must have for a given length of truss to sustain a
specified weight.

There are rules for all these problems, some of them very intricate, but
all of them intensely interesting. It will be a valuable addition to
your knowledge to give this subject earnest study.

CHAPTER XIII

THE BEST WOODS FOR THE BEGINNER

In this place consideration will be given to some of the features
relating to the materials to be employed, particularly with reference to
the manner in which they can be worked to the best advantage, rather
than to their uses.

THE BEST WOODS.--The prime wood, and the one with which most boys are
familiar, is white pine. It has an even texture throughout, is generally
straight grained, and is soft and easily worked. White pine is a wood
requiring a very sharp tool. It is, therefore, the best material for the
beginner, as it will at the outset teach him the important lesson of
keeping the tools in a good, sharp condition.

SOFT WOODS.--It is also well for the novice to do his initial work with
a soft wood, because in joining the