Long before hydro power began providing electricity, it was used to perform simple but labour-intensive tasks. There is no consensus among historians regarding when and where the earliest hydraulic technology emerged. It is clear, however, that water power was used extensively in the Roman Empire and eastern Mediterranean region by at least the first century BCE. At this time, water-powered technology took the form of waterwheels—bladed wheels that sat on either a horizontal or vertical axis and transferred the kinetic energy of moving water into mechanical energy, most often to turn a millstone to grind grain into flour. The simplest in design was the horizontal waterwheel, which was usually positioned directly underneath the floor of a mill. The wheel sat horizontally in flowing water and was connected to the millstone by a simple vertical axle that turned as the water pushed the blades of the wheel; the turning wheel would thus rotate the millstone.
The simplicity of its design made the horizontal waterwheel relatively inexpensive and easy to operate. However, it provided much less power than the more common vertical waterwheels, which sat upright and transferred the kinetic energy of the water into mechanical energy through a series of gears and wheel-shafts. The first type of vertical waterwheel was the undershot model, which stood
upright with the bottom portion submerged in the river or stream. The natural flow of the water would provide the energy necessary to turn the blades, much like a horizontal waterwheel. The undershot wheel, however, was much more efficient at transferring the kinetic energy of the water into mechanical energy. The primary disadvantage of the undershot wheel was that it depended on the steady, consistent flow of the river or stream. If water levels were too low, there would not be sufficient energy to turn the wheel; conversely, too much water would submerge the wheel and render it useless.
These problems were largely solved by the second variant of vertical waterwheel, the overshot wheel. Construction of an overshot waterwheel typically involved the diversion of water from a river through an aqueduct, thus allowing the operator more control over regulating the flow of the water. The water was delivered to the wheel through a flume, and the wheel was turned at once by the flow and weight of the falling water. Overshot wheels were more expensive to build because they required water to be diverted from a stream or river; however, because they gained the additional advantage of gravity (the weight of the falling water), overshot wheels were much more powerful than undershot or horizontal models.