The flow through the hose is limited both by how hard it is to push the water through the hose and how hard it is to push it out the nozzle. The nozzle is much shorter than the hose, so when open it provides only a little bit of the resistance to flow. As you start to narrow its opening its resistance goes up, but not enough to slow the total flow much, because the hose resistance is still bigger.
Now let's look at the water flow as it leaves the nozzle. When the nozzle is wide open (say 1 cm2
opening) a flow of 1000cm3
/second would require the exit velocity to be 1000 cm/s. If the nozzle is narrowed to say 0.5 cm2
the flow will drop a little (maybe to 900 cm3
/s) but the exit velocity now must be 900/0.5=1800 cm/s. So the stream will go farther.
This description is a little over-simplified, but I think you get the idea.
(published on 10/18/2008)