» For the standard botanical author abbreviation Torr., see John Torrey. The torr (symbol: Torr) is a non-SI unit of pressure defined as 1/760 of an atmosphere. It was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644. History Torricelli attracted considerable attention when he demonstrated the first mercury barometer to the general public. He is credited with giving the first modern explanation of atmospheric pressu… (More on Torr)