48 PHYSICS. 
that of efflux. Upon this principle depends the efficacy of Segner’s water 
wheel. This consists of a vessel capable of turning about a vertical axis, 
at whose foot is a horizontal tube, bent in opposite directions at the two 
extremities, and in the same horizontal plane. The water escaping through 
these extremities produces a rapid rotation by the reaction of pressure on 
the sides of the tube opposite the opening; provided, however, that the 
pressure be sufficient to overcome the friction. 
If a stream of water be directed against a movable body, it will cause a 
change in its position; and the force with which this is done will be in pro- 
portion to the amount of pressure. If, during the unit of time, as one second, 
a stream of water, whose height is M, fall from a height, h, Mh will be the 
momentum of this column of water; and the force obtained by the impact 
of the water may be easily calculated. 
The most important application of the impact of water is to be found in 
‘water wheels used for the propulsion of machinery. The most usual water 
wheels are vertical, with a horizontal axis. They are divided, according to 
the point of application of the force, into overshot, in which the water falls 
into the buckets of the wheels, from above and beyond the highest point ; 
undershot, in which the water strikes against the lower float boards; and 
middleshot, or a medium between the other two. In the ordinary water 
wheels a good deal of power is lost; Poncelet has therefore constructed wheels 
with curved floats, which are much more powerful. Most powerful of all, 
however, are the so-called top-wheels, or turbines, invented by Fourneyron. 
In these the wheel is horizontal and the floats vertical; the water is carried 
through peculiarly constructed conducting curves against the floats, and 
turns the wheel around like a top, with such force indeed that 75—80 per 
cent. of the force of water employed is effective. In the division of the work 
specially devoted to Technology and Machinery, reference will again be 
made to the technical application of water power ; where also the construc- 
tion of the water-column machine will be explained—a machine in which 
the pressure of the water acts upon the piston of a pump, producing a back- 
ward and forward motion, which can be transmitted by proper connexions 
to other machinery. We may mention, in conclusion, another hydraulic 
machine, which can be employed to great advantage in many cases: this is 
the hydraulic ram, invented by Montgolfier in 1797, and employed in raising 
water. In pl. 17, fig. 36, mm is a horizontal tube, in which the water flow- 
ing from a reservoir moves with a velocity dependent upon the height of 
pressure. At kis a valve closed by the velocity of the escaping water ; by 
it the aperture at this place may be closed. The water now pressing 
through the tube 7 into the cast iron reservoir d, enters, after raising another 
valve, into a great cast iron receiver (the air-vessel), and in this manner 
reaches the ascent tube, ca. Into this it is driven with a much greater 
force than would be produced by the height of pressure alone, as by the 
closing of the first valve, which suddenly obstructs the motion of the water 
escaping there, a pressure is produced upon the sides of the tube. In the 
ascending tube, the water rises to the height allowed by the elasticity of the 
air in the air-vessel, and the pressure of the water already raised ; then the 
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