There was a BBC program showing how the Romans designed their water systems, absolutely bloody amazing!!
these long aqueducts only dropped one inch over a mile, one inch!!, they had to be that precise, and they could do that back then!!

then when the water came to Pompei, it filled a small pool/reservoir, and it had 3 outlets, the one with deepest outlet was the main outlet for public fountains/drinkwater and road cleaning, so it always flowed in priority due to having a deeper channel, another went to public baths and last to private mansions which was the first to be shut or dry up when water was low.

the water went down a steep hill, it's force would thus be so fierce it could break a man's arm
so, they came up with a way to over come that
every so often on the street, there's be a 10' square brick tower going up 20+ feet, on top would be a bronze water cistern
so, the high pressure water would rocket up pipes on the tower,and go into the cistern, thus losing force as it climbed
the water would then flow out of the cistern into fountains around the tower, so folk could get water, and then overflowed into the roads, which would thus be swept clean by the water which ran all the time. 

Roman engineering was bloody awesome! :)
old video I did with Bryce andmodel I made, showing how a ballista worked:
http://www.silverblades-suitcase.com/movies/ballista_light.avi

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