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1804 was a leap year starting on Sunday of the Gregorian calendar and a leap year starting on Friday of the Julian calendar, the 1804th year of the Common Era (CE) and Anno Domini (AD) designations, the 804th year of the 2nd millennium, the 4th year of the 19th century, and the 5th year of the 1800s decade. As of the start of 1804, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- October 5 – War between Spain and the United Kingdom is triggered by the battle between four British warships (the Indefatigable, the Medusa, the Amphion and the Lively) and four Spanish frigates (Medee, Fama, Clara and Mercedes), all carrying treasure and merchandise. Captain Graham Moore of the Indefatigable informs Spanish Admiral Jose Bustamente of his orders to detain the treasure-laden ships and, "not receiving a satisfactory answer, an Action commenced"; La Mercedes is sunk and the other three ships surrender.
- October 8 – Jean-Jacques Dessalines holds his coronation as Jean-Jacques I, Emperor of Haiti.
- November 3 – The Treaty of St. Louis is signed by Quashquame and William Henry Harrison; controversy surrounding the treaty eventually causes the Sauk people to ally with the British during the War of 1812, and is the main cause of the Black Hawk War of 1832.
- November 20 – Said bin Sultan, Sultan of Muscat and Oman, starts to rule.
- November 30 – The Democratic-Republican-controlled United States Senate begins an impeachment trial against Federalist-partisan Supreme Court of the United States Justice Samuel Chase, on charges of political bias (he is acquitted by the United States Senate of all charges on March 1, 1805).
- December 2 – Coronation of Napoleon I: At the cathedral of Notre Dame de Paris, Napoleon crowns himself as the first Emperor of the French in a thousand years. Witnessing this, Simón Bolívar dedicates himself to liberating Venezuela from Spanish rule.
- December 3 – Thomas Jefferson defeats Charles C. Pinckney in the United States presidential election.
- December 12 – Spain declares war on the United Kingdom.
- January 1 – James Fannin, Texas revolutionary (d. 1836)
- January 10 – Élie Frédéric Forey, Marshal of France (d. 1872)
- January 20 – Eugène Sue, French novelist (d. 1857)
- January 21 – Eliza Roxcy Snow, American poet (d. 1887)
- February – James Bronterre O'Brien, Irish-born Chartist (d. 1864)
- February 7 – John Deere, American industrialist (d. 1886)
- February 13 – Claude-Étienne Minié, French army officer and weapon inventor (d. 1879)
- February 29 – Carl von Rokitansky, Czech physician and pathologist (d. 1878)
- March 8 – Alvan Clark, American telescope manufacturer (d. 1887)
- March 14 – Johann Strauss Senior, Austrian composer (d. 1849)
- March 17 – Jim Bridger, American trapper and explorer (d. 1881)
- March 20 – Neal Dow, mayor of Portland and Father of Prohibition (d. 1897)
- April 3 – Lucien Baudens, French military surgeon (d. 1857)
- April 4 – Andrew Nicholl, Northern Irish painter (d. 1886)
- April 26 – Charles Goodyear, American politician (d. 1876)
- May 4 – Margaretta Riley, British botanic (d. 1899)
- May 16 – Elizabeth Peabody, Transcendental activist, educator (d. 1894)
- June 1
- June 24 – Willard Richards, American religious leader (d. 1854)
- July 4 – Nathaniel Hawthorne, American writer (d. 1864)
- July 6 – Jerónimo Carrión, 8th President of Ecuador (d. 1873)
- July 14 – Ludwig von Benedek, Austrian general (d. 1881)
- July 20 – Richard Owen, English anatomist, paleontologist, and zoologist (d. 1892)
- July 23 – Jane Irwin Harrison, de facto First Lady of the United States (d. 1846)
- July 28 – Ludwig Feuerbach, German philosopher (d. 1872)
- September 5 – William Alexander Graham, United States Senator from North Carolina (1840-1843), Confederate States Senator (1864-1865) (d. 1875)
- September 8 – Eduard Mörike, German poet (d. 1875)
- September 11 – Mercedes Marín del Solar, international Chilean poet and reform educator (d. 1866)
- September 14
- October 18 – Mongkut, Rama IV, King of Siam (d. 1868)
- October 24 – Wilhelm Eduard Weber, German physicist (d. 1891)
- November 18 – Alfonso Ferrero La Marmora, Italian general and statesman (d. 1878)
- November 23 – Franklin Pierce, 14th President of the United States (d. 1869)
- December 7 – Noah Haynes Swayne, Associate Justice of the Supreme Court of the United States (d. 1884)
- December 10 – Carl Gustav Jacob Jacobi, German mathematician (d. 1851)
- December 13 – Joseph Howe, Canadian politician (d. 1873)
- December 16 – Viktor Bunyakovsky, Ukrainian-Russian mathematician (d. 1889)
- December 21 – Benjamin Disraeli, Prime Minister of the United Kingdom (d. 1881)
- December 23 – Charles Augustin Sainte-Beuve, French literary critic (d. 1869)
- January 4 – Charlotte Ramsey Lennox, British author and poet (b. 1727)
- January 15 – Dru Drury, English entomologist (b. 1725)
- February 6 – Joseph Priestley, English chemist (b. 1733)
- February 12 – Immanuel Kant, German philosopher (b. 1724)
- March 3 – Giovanni Domenico Tiepolo, painter (b. 1727)
- March 13 – Damodar Pande, Prime Minister of Nepal (b. 1752)
- March 16 – Henrik Gabriel Porthan Finnish writer and historian (b. 1739)
- March 21 – Louis Antoine, Duke of Enghien (executed) (b. 1772)
- March 30 – Victor-François, 2nd duc de Broglie, Marshal of France (b. 1718)
- April 9 – Jacques Necker, French statesman (b. 1732)
- April 11 – Miklós Küzmics, Hungarian Slovenes writer, Catholic priest (b. 1737)
- April 15 – Jean-Charles Pichegru, French general (strangled in prison) (b. 1761)
- July 12 – Alexander Hamilton, American statesman (killed in a duel) (b. 1755)
- September 4 – Richard Somers, American naval officer (killed in battle)
- October 2 – Nicolas-Joseph Cugnot, French steam vehicle pioneer (b. 1725)
- October 8 – Thomas Cochran (judge), Canadian judge (b. 1777)
- November 5 – Maria Anna Adamberger, Austrian actress (b. 1752)
- November 18 – Philip Schuyler, general in the American Revolution, a United States Senator from New York, father of Angelica Schuyler Church and Elizabeth Schuyler Hamilton (b. 1733)
- November 23 – Richard Graves, English writer (b. 1715)
- December 18 – Jacob ben Wolf Kranz, Lithuanian maggid (b. c. 1740)
- December 25 – Contarina Barbarigo, famous Venetian noble.
- ^ Rattenbury, Gordon; Lewis, M. J. T. (2004). Merthyr Tydfil Tramroads and their Locomotives. Oxford: Railway and Canal Historical Society. ISBN 0-901461-52-0.
- ^ Gaffield, Julia (2015). Haitian Connections in the Atlantic World: Recognition after Revolution. Chapel Hill: University of North Carolina Press. pp. 83–84.
- ^ Whitaker, Anne-Maree. "Castle Hill convict rebellion 1804". Dictionary of Sydney. Retrieved 2013-03-03.
- ^ Penguin Pocket On This Day. Penguin Reference Library. 2006. ISBN 0-14-102715-0.
- ^ "Our timeline". Bible Society. Retrieved 2010-11-26.
- ^ "The Eighth Group of the Periodic System and Some of its Problems", by James Lewis Howe, in The Chemical News and Journal of Physical Science (July 20, 1900) p31
- ^ Nicholas Harris Nicolas, The Dispatches and Letters of Vice Admiral Lord Viscount Nelson (Cambridge University Press, 1846; reprinted 2011) p266
- ^ John Relly Beard, The Life of Toussaint L'Ouverture, the Negro Patriot of Hayti (James Redpath Co., 1863, reprinted by University of North Carolina Press, 2012) p271
- ^ "Luce Ben Aben School of Arab Embroidery I, Algiers, Algeria". World Digital Library. 1899. Retrieved 2013-09-26.