HYDRAULICS. 



propelled through the forcing valves, D D; and the enclosed air being 4 compressed, 

 re-acts upon the water, which is projected up the centre pipe and along the hose 

 with a force proportioned to the power applied to the pistons by the persons 

 who work the engine. 



The fourth class of hydraulic machines for raising water consists of such 

 engines as act either by the weight of a portion of the water which they have to 

 raise, or of the water, or by its centrifugal force, momentum, or other natural 

 powers. The centrifugal pump (fig. 9) belongs to this class. This machine 

 raises water by means of the centrifugal force, combined with the pressure of 

 the atmosphere. A B is an upright spindle, so fixed that rapid rotary motion 

 may he communicated to it by a wheel and pinion, or winch, c D, c D, repre- 

 sent any number of curved pipes BO disposed and fixed to the spindle that their 

 lower ends may be near to 't, and be covered by the water to be raised, and their 

 upper ends to be extended to a considerable distance from the centre of motion, 

 and bent downwards to prevent the scattering of the water. Before putting the 

 machine in action the several pipes must be rilled with water, which will be re- 

 tained in them by a valve near the bottom opening upwards. The machine is 

 then put in motion by turning the spindle rapidly. The upper ends of the pipes 

 will describe a much larger circle than the ends below, and, consequently, such a 

 centrifugal force will be generated at the upper ends as will produce a vacuum, 

 and the water below will then rise and flow from the upper ends of the pipes 

 into the circular trough, whence it may be delivered by a pipe as required. 



Power to be derived from Water- Motion is generally obtained from 

 water either by exposing obstacles to the action of its current, as in water-wheels, 

 or by arresting its progress in raoveable buckets or receptacles, which retain it 

 during its descent A water-wheel consists of a hollow cylinder or drum, re- 

 volving on a central axle, from which the power is communicated ; while the 

 exterior surface is occupied by float-boards, or cavities upon which the water is 

 to act. Water-wheels are divided into three classes first, the Undershot wheel 

 (fig 10), having floats dipping into the water, the current of which, acting 

 against them, causes the wheel to revolve. The second 'class are those termed 

 Overshot wheels (fig 11), in which the circumference is occupied by a series of 

 cavities, into which the water falls from above ; as the wheel revolves these 

 cavities become inverted and discharge their contents at the bottom of the 

 wheel. This description of wheel is much more powerful, as well as much more 

 economical in its consumption of water, than the preceding. The third kind of 

 watei'- wheel is that termed the Breast wheel. In this the water is delivered 

 about half-way up it, or rather below the level of the axis, and the brickwork 

 upon which the water descends is built in a circular form, so as to make it 

 parallel to the edges of the float-boards of the wheel ; its arrangement and mode 

 of action will be readily understood by reference to fig. 12. 



Fi. 13 represents Barker's mill, a machine which owes its efficacy to the 

 centrifugal force. It consists of a long cylindrical pipe, having a funnel at A, 

 and terminating in a pivot, turning in a socket, at B. About A is an axis c, 

 passing through a frame, and carrying with it the upper millstone. At the 

 bottom of the pipe at B, is a cross pipe, D E, at the opposit- 1 sides of which arc t\vo 

 aperture;', from which the water poured into the funnel at A, spouts with 

 considerable velocity, and, from the resistance of the air, gives motion to the 

 machine. 



