CHAPTER 5.

Extending the Time of Action of Forces

45. This is one of the most common and most useful of the
employments of machinery. The half minute which we daily devote
to the winding-up of our watches is an exertion of labour almost
insensible; yet, by the aid of a few wheels, its effect is spread
over the whole twenty-four hours. In our clocks, this extension
of the time of action of the original force impressed is carried
still further; the better kind usually require winding up once in
eight days, and some are occasionally made to continue in action
during a month, or even a year. Another familiar illustration may
be noticed in our domestic furniture: the common jack by which
our meat is roasted, is a contrivance to enable the cook in a few
minutes to exert a force which the machine retails out during the
succeeding hour in turning the loaded spit; thus enabling her to
bestow her undivided attention on the other important duties of
her vocation. A great number of automatons and mechanical toys
moved by springs, may be classed under this division.

46. A small moving power, in the shape of a jack or a spring
with a train of wheels, is often of great convenience to the
experimental philosopher, and has been used with advantage in
magnetic and electric experiments where the rotation of a disk of
metal or other body is necessary, thus allowing to the enquirer
the unimpeded use of both his hands. A vane connected by a train
of wheels, and set in motion by a heavy weight, has also, on some
occasions, been employed in chemical processes, to keep a
solution in a state of agitation. Another object to which a
similar apparatus may be applied, is the polishing of small
specimens of minerals for optical experiments.