343. In the lectures on the mechanical powers which have been just
completed, we have seen how great weights may be raised or other large
resistances overcome. We are now to consider the important subject of
the application of mechanical principles to _structures_. These are
fixtures, while machines are adapted for motion; a roof or a bridge is
a structure, but a crane or a screw-jack is a machine. Structures are
employed for supporting weights, and the mechanical powers give the
means of raising them.
344. A structure has to support both its own weight and also any load
that is to be placed upon it. Thus a railway bridge must at all times
sustain what is called the permanent load, and frequently, of course,
the weight of one or more trains. The problem which the engineer solves
is to design a bridge which shall be sufficiently strong, and, at the
same time, economical; his skill is shown by the manner in which he can
attain these two ends in the same structure.
345. In the four lectures of the course which will be devoted to
this subject it will only be possible to give a slight sketch, and
therefore but few details can be introduced. An extended account of
the properties of different materials used in structures would be
beyond our scope, but there are some general principles relating to the
strength of materials which may be discussed. Timber, as a building
material, has, in modern times, been replaced to a great extent by iron
in large structures, but timber is more capable than iron of being
experimented upon in the lecture room. The elementary laws which we
shall demonstrate with reference to the strength of timber, are also,
substantially the same as the corresponding laws for the strength
of iron or any other material. Hence we shall commence the study of
structures by two lectures on timber. The laws which we shall prove
experimentally will afterwards be applied to a few simple cases of
bridges and other actual structures.