how mechanical clock work?

Question

for spring driven clock

Answer 1

Mechanical clocks have a power supply.
Usually a wound up spring. (Grandfather clocks usually have a weight on a string)
This energy is regulated by an oscillator (from the Greek "to swing") and escapement.
Galileo Galelei noticed that the swinging of a lamp was very regular despite the distance swung through.
A balance wheel works on the same principle.
http://en.wikipedia.org/wiki/Mechanical_Watch

Answer 2

Perfectly, twice a day, when they stop! Sorry,couldn't resist.