Horizontal mills exploit water power for various tasks. Today we are still familiar with similar methods of harnessing energy using water power as waterwheels are still in use.
Generally speaking, a horizontal mill has a 'horizontal' wheel which water is channelled on to via a system of races controlled by a sluice gate. The wheel turns an axle which is attached to a grinding stone or a gear mecchanism of some sort.
Various writers at the end of the Middle Ages describe and illustrate the machinery found in a horizontal mill.
These include Ramelli's 1588 Les Diverse ed Artificioso Machine and Georgius Agricola's De Re Metallica (from 1556). A diagram of a relatively modern mill can be viewed here.
The terminology is also known from medieval sources in Italy as outlined here.
The extensive excavation of an early medieval settlement at Raystown, Co. Meath produced a complex of mill streams and mills.
As can be seen in this image, the base of a mill contains the undercroft with the horizontal wheel and some sort of pivot to keep the axle vertical. The superstructure has normally rotted away long ago and is no longer visible. The wheel has a series of paddles or vanes attached that are hit by the water which enters the undercroft via a flume and drives the wheel around.
Vertical watermills are also known, where the wheel is set upright and is around driven by water hitting the paddles at the bottom (known as undershot) or top (called overshot). At Nendrum, in County Down, a mill was excavated where the water was retained in a mill pond at high tide. This type is known as a tidal mill.
Milling terms in early Irish texts are also known from various sources in Ireland and Scotland and are summarised here: Mac Eoin, G. (1981) The early Irish vocabulary of mills and milling, in Studies on Early Ireland, 13-19. An diagram from Mac Eoin's paper is shown below.
