#16 Stephen Van Rensselaer
Peak fortune: $68.5
Source of wealth: Major of the US Militia and member of the New York State Assembly, Rensselaer was also heir to one of the most well-endowed estates in the US. He founded the Rensselaer Polytechnic Institute with a portion of his wealth.
#15 Jay Gould
Peak fortune: $71.2
Source of wealth: Railroad baron and gold speculator, Jay Gould masterminded the 19th century transportation boom in America. He and financier James Fisk also bought up a dominating share of the gold market at the time - enough to directly affect market movements during Gould's lifetime.
#14 Henry Duke of Lancaster
Peak fortune: $77.5 billion
Source of wealth: English diplomat, noble, soldier, and founding member of the Order of the Garter.. He is remembered today for his peace talks with France and for being a patron of Corpus Christi College, Cambridge
#11 John of Gaunt
Peak fortune: $101 billion
Source of wealth: Richard II's regent during much of the latter's rule and the father of Henry Bolingbroke (who would eventually come back from exile to depose Richard and take the English throne into his hands). One of Shakespeare's meekest characters.
#10 Stephen Girard
Peak fortune: $105 billion
Source of wealth: Successful in the shipping trade, he was a French-born American merchant that went into the banking business later in his life, owning a bank that was called "Girard's Bank".
#9 Richard Fitzalan 10th Earl of Arundel
Peak fortune: $108 billion
Source of wealth: The Earl of Arundel and quite the military leader. He fought in the Scottish Independence wars and in the Hundred Years' War. Subsequent Earls of Arundel -- his sons -- would make this list, but we're only counting that money once.
#8 John Jacob Astor
Peak fortune: $121 billion
Source of wealth: A successful fur trader, he established a near monopoly within the U.S. by around 1800. He subsequently switched trades and went on to investing in real estate, focusing on New York City.
#7 William de Warenne
Peak fortune: $134 billion
Source of wealth: Originally from Normandy, William I de Warenne participated in the battle of Hastings and was rewarded with properties in Sussex, Northfolk and Yorkshire. He became the first Earl of Surrey.
#6 Bill Gates
Peak fortune: $136 billion
Source of wealth: Founded Microsoft with Paul Allen in 1975. He held onto shares as Microsoft dominated the age of computers, peaking in personal wealth at the top of the Dot Com Bubble.
Estimate from wide-spread reports of a $101 billion net worth for a period in 1999, adjusted for inflation.
#5 Alan Rufus
Fortune: $149 billion
Source of wealth: A Norman who joined William The Conqueror in the invasion of Britain in the 11th century, Alan "The Red" had 250,000 acres of land from Yorkshire to London. He also owned Richmond Castle in North Yorkshire, which, for the time, was considered very comfortable.
#4 Cornelius Vanderbilt
Fortune: $185 billion
Source of wealth: In 1862, he began to buy railroad lines. Although already 70 years old, his wealth mostly comes from this business of the 19th century. Prior to that, he was known as a cold-blooded steam-boat entrepreneur.
#3 William The Conqueror
Peak fortune: $209 billion
Source of wealth: Was called 0William the Bastard until he led the last successful foreign invasion of England in 1066. Although he became a monarch, we're counting the spoils of war before he took the throne, based on what he gave out to his sons Odo and Robert.
#1 John D. Rockefeller
Fortune: $336 billion
Source of wealth: He founded Standard Oil in 1870, at the age of 31, and bought up most of the oil refineries in the United States, eventually controlling about 90% of the American oil business.
Bonus: Some people who didn't make the list
Marcus Licinius Crassus: Often named as the richest man ever, a more accurate conversion of sesterce would put his modern figure between $200 million and $20 billion. Great discussion here.
The Rothschilds: Scholars put the Rothschild fortune during the 19th century at £400,000,000 in 1961 money. Converted to dollars and adjusted for inflation, they don't make the list. (Portrait Of A Dynasty, page 57)
Genghis Khan: Owned most of Asia, but we couldn't find an estimate.