s
the relation which Kepler bad discovered between the relative motions of
the planets. Every such discovery coming in this age of the rejuvenation
of experimental science had a peculiar force in teaching men the
all-important lesson that simple laws lie back of most of the diverse
phenomena of nature, if only these laws can be discovered.

Galileo further observed that his pendulum might be constructed of
any weight sufficiently heavy readily to overcome the atmospheric
resistance, and that, with this qualification, neither the weight nor
the material had any influence upon the time of oscillation, this being
solely determined by the length of the cord. Naturally, the practical
utility of these discoveries was not overlooked by Galileo. Since a
pendulum of a given length oscillates with unvarying rapidity, here is
an obvious means of measuring time. Galileo, however, appears not to
have met with any great measure of success in putting this idea into
practice. It remained for the mechanical ingenuity of Huyghens to
construct a satisfactory pendulum clock.

As a theoretical result of the studies of rolling and oscillating
bodies, there was developed what is usually spoken of as the third law
of motion--namely, the law that a given force operates upon a moving
body with an effect proportionate to its effect upon the same body when
at rest. Or, as Whewell states the law: "The dynamical effect of
force is as the statical effect; that is, the velocity which any
force generates in a given time, when it puts the body in motion, is
proportional to the pressure which this same force produces in a body
at rest."(2) According to the second law of motion, each one of the
different forces, operating at the same time upon a moving body,
produces the same effect as if it operated upon the body while at rest.

STEVINUS AND THE LAW OF EQUILIBRIUM

It appears, then, that the mechanical studies of Galileo, taken as a
whole, were nothing less than revolutionary. They constituted the first
great advance upon the dynamic studies of Archimedes, and then led to
the secure foundation for one of the most important of modern sciences.
We shall see that an important company of students entered the field
immediately after the time of Galileo, and carried forward the work he
had so well begun. But before passing on to the consideration of their
labors, we must consider work in allied fields of two men who were
contemporaries of Galileo and who