gravity. In 1834, the Houses of Parliament burned down so a new set of national standards of length were commissioned. In 1835 George Airy became Astronomer Royal and commissioned new instruments. Large castings were contracted to Maudslay and Ransomes. Optical parts were often supplied by Dollond. From about 1830 to 1850 Britain was gripped by Railway Mania resulting in high demand for levels and theodolites. Troughton & Simms exhibited at the Great Exhibition in 1851. William Simms died in 1860 and the firm took William Simms Junior and his cousin James into partnership. Optical glass technology allowed larger lenses to be made and these were sourced from French and German makers such as Guinand and Merz. In 1866, a transit and spectroscope were ordered by Harvard College Observatory and delivered in 1870. Slow delivery appears to have happened a lot and seems to have been partly due to insistence on precision but also the volume of business being undertaken. In 1860, Troughton & Simms opened a new works at Charlton. There were many requests from around the world for standard measure bars, and in 1876, they supplied the Imperial Standards Of Length gauges mounted at Trafalgar Square and the Greenwich Royal Observatory. The firm produced hundreds of astronomical instruments such as mural circles, transit circles, sextants, and other astronomical instruments for observatories around the world. Towards the end of this period other countries such as France, Germany and the United States were able to make instruments themselves so Troughton & Simms made more of their product for the British market. Notable instruments from this period include the Troughton & Simms Altazimuth Refractor (1847) at Greenwich Royal Observatory and the Troughton & Simms Astronomical Telescope (1880) for Tokyo Astronomical Observatory. Troughton & Simms in the late 19th and early 20th centuries By 1887, Troughton & Simms employed 200 people at their Charlton works. Military production began to be important with production of coincidence rangefinders for the British government. Among their clients were the Italian Navy, the Russian government and the Austro-Hungarian Hydrographic Department Observatory. Undated advertisements from this period include the claim that "in fifteen years our output of levels and theodolites alone was 23,000". An indication of their worldwide sales is how often instruments appear all around the world. Troughton & Simms did not normally place serial numbers on their products, so dating them is inexact. The "Troughton & Simms"