MECHANICS. 47 
degree. In streams directed upwards, the jet expands continually after it has 
reached its greatest contraction of two thirds of its diameter, at a distance 
from the orifice equal to its diameter. The stream retains its constant form 
during only a certain part of its length; then it is separated into greater or 
smaller currents, which assume very various forms according to the shape 
of the orifice of efflux. 
Should the efflux take place not through a thin plate but through a tube, 
considerable changes take place if the tube have not the shape of the com- 
pressed stream of water. Cylindrical escape pipes do not produce any 
difference under great pressure; at a less pressure, however, they increase 
the discharge, this taking place to a still greater extent in conical pipes: in 
all these cases, however, the velocity of efflux is diminished. 
b. Lateral Pressure. 
Pl. 17, fig. 30, illustrates the laws of the lateral pressure of moving liquids. 
If water flow from a vessel, A, through tubes, their sides will experience no 
pressure if there is no friction to overcome, but by this a considerable part 
of the hydrostatic pressure is lost, and acts upon the walls of the tube. The 
narrower the tube, the greater is the friction, and so much less the velocity 
of efflux. The pressure which the walls of a tube, cf, have to experience, 
will be less the nearer to the aperture of efflux, f; making then an aperture 
at c, and erecting in it a vertical tube, the water will ascend to a height, cb, 
corresponding to the pressure on the walls of the tube at this point. Midway 
between ¢ and f, at e, the pressure on the walls is only half as great; the 
water would therefore rise only half as high as at c, namely, to d; and 
placing in any other part, between c and f, a vertical tube, the level of the 
water would lie in the straight line, bf. 
_ To measure the pressure of falling water, the apparatus represented in 
pl. 18, jig. 72, may be employed. Upon the foot, B, stands a cylinder in 
which the post A may be fixed at different heights. DF is a balance beam, 
whose horizontal position may be determined by the index on the graduated 
arc C. At E hangs a common scale-pan, and at F is a plate whose size 
equals that of the efflux orifice of the vessel G. Letting a stream of water 
fall upon F, it will press downwards upon this plate, and the horizontal posi- 
tion of the beam is to be restored by weights placed in EK. These tags 
will represent the pressure of the water. : 
c. Reaction and Impact of Water. 
If a vessel be filled with water, without an aperture in any part of it, 
everything will be in equilibrium ; if, however, in any part of the vessel an 
opening be made and efflux allowed, the pressure ceases at this point, and is 
consequently less than on the part of the vessel diametrically opposite: the 
vessel, then, if allowed, would move in a direction diametrically opposite to 
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