DRAINAGE 
to remove a large part of the rain in 48 
hours. At times the water cannot pass 
through the soil fast enough, even though 
the tiles are large enough to carry it off, 
so that part will need to be removed over 
the surface. 
The total rainfall in different sections 
varies materially. Drainage has to deal 
with extremes of rainfall rather than the 
mean. Laboratory experiments are so 
different from field conditions that our 
best deductions come from the working 
of drains in land of a Known character. 
Generally, if the main drains have the 
capacity to remove one-half inch in depth 
of water from the entire tract in 24 hours, 
they afford what may be regarded as good 
farm drainage. This is the capacity of 
many good systems in alluvial soils. In 
places where no advantages can be taken 
of surface flow, mains may be arranged 
to carry away one inch of water in 24 
hours. 
Where several laterals empty into a 
main, the latter must have a capacity 
nearly equal to their combined flow; but 
it is not possible to calculate the total 
or relative sizes with the exactness 
which is possible with a pressure system 
of pipes. This is due to the effect of the 
soil, which acts as a sponge, and gives up 
its water gradually and to the eddies 
caused by joints. The greater the fall the 
greater the capacity. 
The area of a cross-section of a tile 
increases in ratio of the squares of the 
diameters. Thus 2, 3, 4 and 5 feet tile 
have cross-sectional areas with a ratio 
of 4, 9,16 and 25 square inches. Friction 
and eddies are less in large tile, so that 
doubling the size of tile makes the ca- 
pacity more than four times as great. 
Longer length of tile gives less capacity, 
due to increased friction. In general, a 
4-inch tile will drain about five acres, and 
should not be over 500 or 600 feet long. 
A 5-inch tile will drain 10 acres; 6-inch, 
20 acres; 7-inch, 40 acres; 8-inch, 60 acres. 
Direct Leveling 
The first working principle in drain- 
age is the finding of the differences in 
level of two or more points. 
A level surface is one that is parallel 
903 
to the surface of standing water. A 
water surface is not level theoretically, 
due to the curvature of the earth’s sur- 
face. It is assumed to be level, and per- 
pendicular to a vertical line or the line 
of gravity. Thus a true level line is a 
curved line whose points are all equi- 
distance from the earth’s center, and is 
apparently level. 
A point is above or below another 
point according as it is a greater or less 
distance from the earth’s center. This 
difference is called ‘difference of level’ 
of two points. The height of a point is its 
distance above a given surface, measured 
on a vertical line, and is called its eleva- 
tion. 
Direct leveling depends on three prin- 
ciples: 
1. That the surface of a liquid in re- 
pose is level. 
2. That a vertical line is perpendicular 
to that surface. 
3. A bubble of air confined in a ves- 
sel otherwise filled with liquid will rise 
to the highest point in that liquid. 
In direct leveling two instruments are 
necessary. (1) The “Y” Jevel, which is 
an instrument that can be adjusted so 
as to mark out a horizontal place in any 
direction from a given point. (2) A 
leveling rod, an instrument that can be 
used to measure vertical distances. As 
accessories to the work, we need a tape 
line, or chain, for measuring distances, 
and a set of eleven pins for marking 
points; also some flags. 
Definitions 
A datum line is the base line to which 
the elevation of every point of a series 
is referred. 
Benchmarks are permanent objects 
whose elevations are determined and re 
corded for future reference. 
Turning points are points where the 
bearing of the line changes, and these 
are marked by placing a pin in the hub 
stake used at this point. 
Backsights are readings on points 
whose elevations are known. A backsight 
is taken for the purpose of obtaining a 
new height of instrument. Backsights 
are plus quantities and are to be added. 
