Roman Emperor Valentinian I made his brother, Flavius Valens, his co-emperor.
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The world's first weightlifting championship took place in London, England.
The Treaty of Florence was signed, which ended the war between the French Republic and the Kingdom of Naples.
The Vienna Philharmonic Orchestra performed its first concert.
The strain of swine flu known as H1N1 had its first case in the US.
Frenchman Henri Fabre flew the world's first seaplane in Martinique, France.
