40 
On Water Supplies suited to 
culating the size of the pipe required for a given discharge, the 
least difference of level must be taken, or, in the case above 
quoted, the height from the water-level in the tank, when nearly 
empty, down to the ball-cock. 
The pipe may follow the undulations of the country, provided 
no point rises above the line of uniform slope. 
rig. 2. — Section of Reservoir and Ball-coch in lower tank, showing the 
Hydraulic inclination. 
Thus, if A (Fig. 2) be the reservoir, and c the ball-cock in the 
lower tank, then A C will be the direct hydraulic inclination, and 
no undulation such as B may rise above the line A C, and, 
moreover, it is necessary to fix a small air-cock at the highest 
point of each vertical bend such as B, for the purpose of letting 
out the air, which is very apt to collect in the bend and obstruct 
the flow of the water. 
The pipe may follow any course out of the direct one that 
may be convenient, but the more the pipe deviates from a 
straight line, the longer it will be, therefore the smaller will be 
the hydraulic gradient, and the smaller the discharge. 
The pipes generally used for small water mains are 1-inch, 
1^-inch, and 2-inch bore, usually of galvanised iron, with 
screwed couplings, and 2J-inch attd 3-inch bore of cast iron, 
usually coated with a composition to prevent them rusting. 
The joints of the latter are made with lead or iron borings. 
The 2^-inch pipes are generally made in 6-foot lengths, the 
3-inch in 9-foot lengths ; so that, by reason of the smaller 
number of joints, it is very nearly as cheap to lay 3-inch as 
2i-inch pipe. 
The following is about the price per yard laid, including 
trenching the ground and jointing materials : — 
