July 7 (UPI) -- On this date in history: In 1846, U.S. Navy Commodore J.D. Sloat proclaimed the annexation of California by the United States. In 1865, four people convicted of conspiring with John Wilkes Booth in the assassination of U.S. President Abraham Lincoln were hanged in Washington. In 1898, U.S. President William McKinley signed a joint resolution of Congress authorizing the annexation of Hawaii by the United States. In 1930, construction began on the Giant Boulder Dam, which in 1947