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38 SMT007 MAGAZINE I SEPTEMBER 2019 town to town without consequence. Advances in transportation, however, created a need for time standards [9] . "Prior to the late 19th century, timekeeping was a purely local phenomenon. Each town would set their clocks to noon when the sun reached its zenith each day. A clock- maker or town clock would be the 'offi- cial' time, and the citizens would set their pocket watches and clocks to the time of the town. Enterprising citizens would offer their services as mobile clock setters, car- rying a watch with the accurate time to adjust the clocks in customer's homes on a weekly basis. Travel between cities meant having to change one's pocket watch upon arrival." With the advent of the railroads, in particu- lar, standard time became crucial to the efficient and safe operation of the railroads from station to station. In 1878, Canadian Sir Sandford Flem- ing introduced the global time zone system we use today. As Rosenberg writes [9] , "Sir Fleming's time zones were heralded as a brilliant solution to a chaotic problem worldwide." Of course, the maritime trades had been using longitude-by- Chronometer techniques since the early 1700s when John Harrison developed the H-4 chro - nometer as the first marine timekeeping device accurate enough to use for navigation. While standard time noon might be per- ceived as a relaxing of the standard for actual solar noon, the overall benefits of consistency are generally much more useful than a strict astronomical interpretation of noon, which leads us to standards that shift over time. Standards That Drift Even the greatest attempts at creating sta- ble, consistent standards can face challenges with change over time. For example, recall that the British Weights and Measures Act of 1878 required periodic checks of all cop - ies against the standard. It's a good thing too, as those mandated regular intercomparisons revealed that the yard standard and its cop- ies were shrinking "at the rate of one part per million every twenty years due to the gradual release of strain incurred during the fabrica- tion process [9] ." The international prototype meter was com- paratively stable, however. A great deal of preventative work had been done before the Metre Convention (1875) to ensure a standard as consistent at possible at the time [10] . "The bars were to be made of a special alloy— 90% platinum and 10% iridium—which is significantly harder than pure platinum, and have a special X-shaped Tresca-style cross to minimise the effects of torsional strain during length comparisons. The first castings proved unsatisfactory, and the job was given to the London firm of Johnson Matthey who succeeded in producing 30 bars to the required specification. One of these—No. 6—was determined to be iden- tical in length to the Mètre des Archives and was consecrated as the international prototype metre at the first meeting of the CGPM in 1889. The other bars, duly cali- brated against the international prototype, were distributed to the signatory nations of the Metre Convention for use as national standards. For example, the United States received No. 27 with a calibrated length of 0.9999984 m ±0.2 μm (1.6 μm short of the international prototype)." The Prague Astronomicol Clock is a medieval clock built in 1410 in the Czech Republic.