WATER PRESSURE MACHINES WATER WHEELS. _Hydraulic machinery_ may be broadly divided into 1. Motor machines, and, 2. Pumps. _Water motors_ may be divided into 1. Water wheels, 2. Turbines, and, 3. Water pressure engines. _In hydraulic motor machines_ a quantity of water descending from a higher to a lower level, or _from a higher to a lower pressure_, drives a machine which receives energy from the water and applies it to overcoming the resistances of other machines doing work. _In the next general class_, work done on the machine by a steam engine or other source of energy is employed in lifting water _from a lower to a higher level_. A few machines such as the ram and jet pump _combine the functions of both motors and pumps_. _The subject of water wheels is but a continuation of much that has been illustrated and defined in the historical introduction_ to which is now added the following summary. In every system of machinery _deriving energy from a natural water-fall there exist the following parts_: (1) _A supply channel_, leading the water from the highest accessible level, _to the site of the machine_; this may be an open channel of earth, masonry, or wood, or it may be a closed cast or wrought-iron pipe; in some cases part of the head race is an open channel, part a closed pipe. (2) _Leading from the motor_ there is a tail race, culvert, or discharge pipe delivering the water after it has done its work. (3) _A waste channel_ placed on or at the origin of the head race by which surplus water, in floods, escapes. (4) _The motor itself_, which either overcomes a useful resistance directly, as in the case of a ram acting on a lift or crane chain, or indirectly by actuating transmissive machinery, as when a turbine drives the shafting, belting, and gearing of a mill. With the motor is usually combined regulating machinery for adjusting the power and speed, to the work done. The great convenience and simplicity of water motors has led to their adoption in certain cases, where no natural source of water power is available. In these cases, an artificial source of water power is created by using a steam engine to pump water to a reservoir at a great elevation, or to pump water into a closed reservoir in which there is great pressure. Water flowing from the reservoir through hydraulic engines gives back the energy expended, less so much as has been wasted in friction. Where a continuously acting steam engine stores up energy by pumping the water, while the work done by the hydraulic engines is done intermittently,—this arrangement is considered the most useful.