Farms and Villages. 
39 
that is, springs, lakes, and rivers, I must say a few words about 
pipes. 
Water flowing through pipes meets with considerable resist- 
ance, in consequence of its adhesion to the sides of the pipes 
and the effort necessary to make the particles of water slide over 
each other, or over the surface of the metal ; this is commonly 
called " skin friction," and it varies directly as the length of the 
pipe and directly also as the square of the velocity with which 
the water flows in the pipe, and consequently as the square of 
the quantity which the pipe delivers per minute. To over- 
come this resistance, a certain amount of pressure is necessary, 
and if water be flowing naturally through a pipe, the pressure 
assumes the form of a column of water, the height of which 
must increase with the length of the pipe, or must increase as 
the square of the quantity of water to be discharged increases, 
and this column is measured as the difference of level between 
the water at the upper end and that of the water at the lower 
extremity of the pipe. 
If the water has to be pumped through the pipe, then the 
pump must exert a pressure above the actual height to which 
the water is raised, corresponding to the pressure of the column 
of water which would be necessary to make the water flow 
naturally. Thus a one-inch pipe, 1000 feet long, will discharge 
a little less than 2 gallons per minute, if laid at a slope of one 
foot fall in 200 feet length, or so that its upper end will be 
5 feet higher than the lower ; but to discharge double the quan- 
tity, or nearly 4 gallons per minute, the slope will have to be 
one in 50, and the difference of level between the two ends will 
be 20 feet, or four times as much as before. 
If the length of the pipe be halved, its slope or " hydraulic 
inclination," or " gradient," as it is called, will still have to be 
the same for the same discharge, but as the length is halved, the 
difference of level between the two ends will also be halved. 
A pipe will be under the most favourable conditions if it be 
laid at a uniform slope, but such an arrangement is seldom pos- 
sible. If it run from a reservoir, say, into a service-tank in a 
house over undulating ground, it will start from the bottom of 
the reservoir, and deliver by a ball-cock into the service-tank ; 
in that case the difference of level, under which the pipe is dis- 
charged, will be the vertical height from the ball-cock to the 
water level in the tank, and the hydraulic inclination will be 
this height divided, not by the direct distance between the two 
points, but by the length of the pipe. Thus : suppose the pipe 
to be 1,260 feet long and the difference of level 6 feet, then 
the hydraulic inclination will be — — = 1 in 210. In cal- 
6 
