would advise slaves the state was free and aid them in getting to nearby stations of the Underground Railroad. Sometimes refugee slaves from the South stayed in Pittsburgh, but other times they continued North, including into Canada. Many slaves left the city and county for Canada after Congress passed the 1850 Fugitive Slave Act, as it required cooperation from law enforcement even in free states and increased penalties. From 1850 to 1860, the black population in Allegheny County dropped from 3,431 to 2,725 as people headed to more safety in Canada. The American Civil War boosted the city's economy with increased iron and armament demand by the Union. Andrew Carnegie began steel production in 1875 at the Edgar Thomson Steel Works in North Braddock, Pennsylvania, which evolved into the Carnegie Steel Company. He adopted the Bessemer process to increase production. Manufacturing was key to growth of Pittsburgh and the surrounding region. Railroad lines were built into the city along both rivers, increasing transportation access to important markets. 1900 to present In 1901, J. P. Morgan and attorney Elbert H. Gary merged Carnegie Steel Company and several other companies into U.S. Steel. By 1910, Pittsburgh was the nation's 8th-largest city, accounting for between one-third and one-half of national steel output. The Pittsburgh Agreement was subscribed in May 1918 between the Czech and Slovak nationalities, as envisioned by T. G. Masaryk, concerning the future foundation of Czechoslovakia. The city suffered severe flooding in March 1936. The city's population swelled to more than a half million, attracting numerous European immigrants to its industrial jobs. By 1940, non-Hispanic whites were 90.6% of the city's population. Pittsburgh also became a main destination of the African-American Great Migration from the rural South during the first half of the 20th century. Limited initially by discrimination, some 95% percent of the men became unskilled steel workers. During World War II, demand for steel increased and area mills operated 24 hours a day to produce 95 million tons of steel for the war effort. This resulted in the highest levels of air pollution in the city's almost century of industry. The city's reputation as the "arsenal of democracy" was being overshadowed by James Parton's 1868 observation of Pittsburgh being "hell with the lid off." Following the war, the city launched a clean air and civic revitalization project known as the "Renaissance," cleaning up the air and the rivers. The