42 
On Water Supplies suited to 
3450 feet long, has a fall of 20 feet, how much will it discharge- 
per minute ? The hydraulic inclination will be - 2q = 
nearly. Look along the base for 170, the nearest number, and' 
run the eye up the line till it reaches the curve of the 2-inch 
pipe ; then follow the horizontal line to the scale on the left 
side, and you will find that nearly 12 gallons will be discharged. 
Or, again, suppose you want to know what sized pipe to lay 
under the above conditions to give 5 gallons per minute, find' 
the spot where the 170 vertical line intersects the 5 gallon 
horizontal line, and you will find that the nearest largest pipe- 
will be 1^ inch diameter. By such means as this, every 
problem connected with pipes can be solved. Main-pipe& 
should always have a cock near their origin, so that they can 
be emptied without emptying the reservoir, and at the lower 
end there should be a lateral cock or " wash-out," near some- 
convenient drain or pond, so that the pipe may be emptied 
without making a mess. The lower tank should have a visible 
overflow, in order that, if the ball-cock fail to act, the overflow 
will at once give warning and prevent waste of water ; and as^ 
ball-cocks are very apt to get out of order, it is well to have a 
cock in the main-pipe close behind it, so that the water may be 
readily shut off, or, if the tank be in a turret or in a house, the 
cock may be in any readily accessible place on the ground level. 
It is not necessary to make the cocks of the same diameter as 
the main-pipes, because the resistance offered to water running 
fast through a cock, the length of which is insignificant, is very 
small indeed. The diameter of the mains has to be consider- 
able, in order that the water may flow slowly, and the con- 
sequent frictional resistance be small, but no such restrictions- 
are required in cocks. The following sizes of cocks will be 
found suitable : — 
To discharge about 4 gallons per minute, a s-inch cock. 
a 3 
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Springs, Rivers, and Lakes. 
Of the three sources of supply, springs, rivers, and lakes^ 
springs are perhaps the best and safest, because their ultimate 
source is the rainfall over a large tnict of open country; hence 
the waters are not likely to be polluted, and even such impu- 
rities as may be taken up in passing through the thin upper 
layer of cultivated and manured soil, are neutralized or de- 
