PUMPS 505 
one leading into the delivery pipe, and the other leading into the 
air chamber. ‘To flow into the delivery pipe the entering water must 
_ exert a pressure greater than that due to the head of water in that pipe 
_ in order to overcome the inertia of the column of water and the friction 
in the pipe, but to enter the air chamber the resistance is practically 
only that due to the air pressure in it, and this pressure is only the 
_ static pressure due to the head in the delivery pipe. Hence the water 
_ coming from the pump barrel, taking the path of least resistance, enters 
the air chamber. As the water enters the air chamber the air‘in it is . 
‘compressed and the pressure rises gradually. This gradually increasing 
_ air pressure acts of course on the column of water in the delivery pipe, 
and sets it in motion gradually without shock. Up to the time A 
. (Fig. 808) all the water going into the delivery pipe has come direct 
- from the pump barrel, but the greater portion of the water coming from 
the pump barrel has gone into the air chamber. At the time A the 
velocity has increased until the flow through the delivery pipe is equal to 
the discharge from the pump, and after this a diminishing pressure is 
sufficient to keep up the delivery. The air now forces water from the 
air chamber, and in doing so it increases in volume and falls in pressure. 
At the time B the air pressure has fallen until it just equals the resist- 
ance. Up to this point the driving force on the water has been greater 
than the resistance, and therefore the velocity of the water has been 
increasing, and is now a maximum. After the time B the water con- 
 tinues to flow from the air vessel, although the air pressure is now less 
than the resistance, because of the kinetic energy in the moving water. 
At the beginning of the second delivery stroke tlie water from the pump 
has again two passages open to it. A quantity sufficient to keep up the 
flow at the now reduced velocity will go into the delivery pipe, but to 
send a greater quantity would mean increasing the velocity, and there- 
fore increasing the pressure above that in the air chamber, hence the 
remainder of the water enters the air chamber, and the pressure increases 
gradually. At the time C the air pressure is just equal to the resistance. 
Between B and C the air pressure, which is the driving force, has been 
less than the resistance, and the velocity has therefore been diminishing, 
and will have reached a minimum at C. Between C and D the air 
pressure increases, and at D the flow through the delivery pipe is again 
equal to the discharge from the pump. At E the velocity is again a 
- maximum, and at F the flow through the delivery pipe is again equal to 
the discharge from the pump, and so on. 
It is seen, therefore, that the air chamber makes the flow of water 
through the delivery pipe continuous, and shocks due to sudden changes 
of pressure are eliminated. 
The positions of the points A, B, C, etc., will depend on the char- 
acter of the motion of the bucket or plunger, the volume of air in the . 
air chamber, and the friction in the delivery pipe. 
The volume of the air chamber varies greatly in practice, being from 
two to six times the displacement of the bucket or plunger per stroke, 
and sometimes as much as ten times. 
_ An air chamber is not so necessary on a double-acting pump, but it 
__ is still advantageous, because the velocity of the piston not being uniform, 
the discharge through the delivery valves is not uniform. The capacity — 
