ration. Elasticity likewise is a
topic that may be introduced with more or less emphasis according to
the predilection of the instructor. The moduli of Young and of simple
rigidity lend themselves readily to quantitative laboratory
experiments. Any amount of interesting material may be culled here
from recent investigations of Michelson, Bridgman, and others with
regard to elastic limits, departures from the simple relations,
variations with pressure, etc., for a lantern or demonstration talk in
these connections.

By this time the student should have found himself sufficiently
prepared to take up problems of rotational motion. The application of
Newton's Laws to pure rotations and combinations of rotation and
translation, such as rolling motions, are very many. We would
emphasize here the dynamic definition of moment of inertia, I = Fh/_a_
rather than the one so frequently given importance for computational
purposes, S_mr_^{2}. Quantitative experiments are furnished
by the rotational counterpart of the Atwood machine. Lecture
demonstrations for several talks abound: stability of spin about the
axis of greatest inertia, Kelvin's famous experiments with eggs and
tops containing liquids, which suggest the gyroscopic ideas, and
finally a discussion of gyroscopes and their multitudinous
applications. The book of Crabtree, _Spinning Tops and the
Gyroscope_, and the several papers by Gray in the _Proceedings of the
Physical Society of London_, summarize a wealth of material. If one
wishes to interject a parenthetical discussion of the Bernouilli
principle, and the simplest laws of pressure distributions on plane
surfaces moving through a resisting medium, a group of striking
demonstrations is possible involving this notion, and by simple
combination of it with the precession of a rotating body the boomerang
may be brought in and its action for the major part given explanation.

Rotational motion leads naturally to a discussion of centripetal
force, and this in turn is simple harmonic motion. This latter finds
most important applications in the pendulum experiments, and no end of
material is here to be found in any of the textbooks. The greatest
refinement of experimentation for elementary purposes will be the
determination of "g" by the method of coincidences between a simple
pendulum and the standard clock. Elementary analysis without use of
calculus reaches its culmination in a discussion of forced vibrations
similar to