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 r
quire 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.