In what year was the pitchers 'mound' introduced?

The pitcher's mound was introduced in 1893. Prior to 1893, there was a 4 foot wide by 5 1/2 foot long box on flat ground that the pitchers pitched from. The pitcher could put his back foot anywhere along the 4 footback line of the box, which was 55 1/2 feet from home plate, to start his delivery. In 1893, to create more offense, the box was replaced with a raised mound and a rubber slab from where the pitcher could start his delivery. The rubber slab was 12 inches in length and positioned 60 1/2 feet from home plate. With the pitched ball having to travel an extra five feet and the pitcher not being able to use a wide angle to deliver the pitch, the league batting average rose 35 points in 1893 and another 29 points in 1894.