«A clock one thousand times more accurate than the atomic clocks used in England to determine an accepted international standard of time has been developed by scientists in Perth, Western Australia. Known as the sapphire clock, its cost is about $200,000, and several have already been built. It can measure one fleeting femtosecond, which is one millionth of a billionth of a second! Of what use is it? According to Einstein’s general theory of relativity, time runs faster the higher one is above the earth. “Our goal is to measure the difference in speed over a height of about one metre—in other words between your feet and your head,” said physicist David Blair, who worked on developing the clock. However, its stability lasts for only five minutes at a time.»
Awake!, October 8, 1996, p.29