 Complete the following phrase:  Britain and Denmark appear to be the only nations that for a short time based their units of length on the pendulum. In 1821 the Danish inch was defined as 138 of the length of the mean solar seconds pendulum at 45 latitude at the meridian of Skagen, at sea level, in vacuum. The British parliament passed the Imperial Weights and Measures Act in 1824, a reform of the British standard system which declared that if the prototype standard yard was destroyed, it would be recovered by defining the inch so that the length of the solar seconds pendulum at London, at sea level, in a vacuum, at 62 F was 39.1393 inches. This also became the US standard, since at the time the US used British measures. However, when the prototype yard was lost in the 