Letter Rate 2¢ Package Rate 2¢ Postcard Rate 1¢ Effective 7/1/1919 United States 6/24/1930 First Airplane Detected Using Radio Waves The first detection of an airplane by using reflected radio waves was accomplished by the US Naval Research Laboratory engineers in Anacostia, Washington, DC. This was a precursor to modern radar.United States 7/7/1930 Hoover Dam Construction Begins Construction of the Hoover Dam began. It took over five years and a total of over 21,000 workers to finish it. It was the largest dam of its time and is one of the largest manmade structures in the world. American President Herbert Hoover (3/4/1929-3/4/1933) British Prime Minister Ramsay MacDonald (6/5/1929-6/7/1935) German President Paul von Hindenburg (5/12/1925-8/2/1934)