Farms and Villages. 
51 
than the diameters of the wheels, provision must be made for 
their stoppage during wet weather by securing storage room 
enough in the service reservoir, and these precautions are at 
times necessary, as I have already pointed out, in order to 
obtain clear and colourless water. 
There are, however, hydraulic motors, which will work under 
moderate falls, and which are not affected by submergence, pro- 
vided the fall remains unaltered. Turbines and reaction wheels 
belong to this class. A turbine consists of a wheel, formed of 
a pair of rings, between which are fitted paddles or blades, 
curved to a particular form. The wheel is usually mounted on 
a vertical axis and has the water admitted into it, either from 
above or from below, by means of a pipe, expanding into a pair 
of rings, the outer diameter of which is a little less than the 
inner diameter of the wheel, but similar to it in other respects, 
and also fitted with curved blades or directrices, by which the 
water flowing out gets a proper direction given to it, and forms 
a series of jets, issuing tangentially all round the inlet pipe, in 
the plane of the wheel. These jets impinge on the blades of 
the wheel, and by their impact cause it to revolve. 
In one modification of this arrangement, known as the Jouval 
turbine, the water, instead of flowing at right angles to the axis, 
issues parallel to it, the revolving wheel being placed in line with 
the fixed ring of directrices. Motors of this kind yield a very 
good efficiency, for even small ones may be expected to give 
60 per cent. 
The reaction-wheel, or " Barker's Mill," as it is often called, 
is a very simple machine, and consists of a pair of hollow 
arms curved to a particular form, fixed to a spindle and capable 
of revolving with it. The water enters from one side in line 
with the axis, and, dividing into two streams, issues as a 
continuous jet from the two revolving arms which are caused 
to turn in the opposite direction to the jets, partly by the 
unbalanced pressure at the open ends, and partly by the 
reaction due to the water being forced out of the straight 
path, in which it would naturally flow, by the curved arms. 
These mills will yield about 40 per cent. duty. All the 
motors belonging to the turbine family work best when com- 
pletely submerged ; they also run at a high velocity, the best 
speed of the periphery of the wheel being, as in the Poncelet 
wheel, about half that due to the fall. Thus, suppose a fall of 
4 feet and a wheel 2 feet diameter. The velocity acquired in 
falling 4 feet is 16 feet a second, hence the speed of the wheel 
should be 8 feet per second, and as the circumference is 6j feet, 
8 ft 
the number of revolutions would be = about 14- per 
Oj It. * ^ 
E 2 
