Horse power.
Smil
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Department of Geography, University of Manitoba, Winnipeg, Canada.
D000383:Agriculture / Q000266:history*; D000818:Animals; D001824:Body Constitution; D049670:History, 17th Century; D049672:History, 19th Century; D049673:History, 20th Century; D049691:History, Medieval; D006736:Horses; D014481:United States