The Royal Institution, London Annual Christmas Lectures to Young People
by Professor George Porter

The earth, whose rotation we use to divide our time into days, is not a very accurate timekeeper. The smaller things are, the faster they move, or oscillate, and the shorter the intervals of time they tick away. We can use atoms to make clocks which are accurate to less than a second in a thousand years, but we can also demonstrate some remarkably accurate chemical and biological clocks.
(Fourth lecture: Wednesday. 6.30 pm)