THE FLOW OF WATER IN CONCRETE PIPE. 75 
must be done from the inside alone under the shadows and imperfec- 
tions of artificial lighting. 
For all shapes, circular or otherwise, the invert is usually laid first. 
Forms for the remainder of the section are then placed on the invert 
and the section completed. Sometimes the first pouring extends just 
across the bottom and sometimes is carried up to the spring line. 
This depends on shape and size of the section. Smooth joints 
between the pourings and between the ends of adjoining set-ups are 
difficult to attain. Unless very heavily braced, the pressure of the 
head of wet concrete will spring the forms and result in offsets. On 
the Catskill aqueduct this amounted to as much as 0.15 foot. 1 The 
writer has seen cases where contractions of from 2 to 4 inches were 
made in sections 5 feet or less in diameter. Other factors that influ- 
ence this movement are weather, length of sections, rapidity of filling 
the forms, and the type of aqueduct. 1 
Before the advent of steel forms, engineers anticipated a rough 
interior when the forms were stripped and relied on a plaster coat to 
fill interstices, estimates of cost being based on that of a plaster coat 
on a certain sized conduit equated against that of an unplastered 
surface on a larger conduit. 2 The present practice in general is to 
use smooth steel forms; clean them thoroughly after each casting, 
then dry and oil or grease them before using again. If close attention 
is given to spading the mixture into close contact with the forms, then 
a surface is secured that is smoother than will be attained by adding a 
plaster coat, local roughness alone being improved by pointing up 
and chipping down. The crown in particular is liable to be rough. 1 
Experience indicates that a given surface is more assuredly repeated 
in a pipe of precast sections than of monolithic construction. Thus a 
smaller factor of safety in the way of capacity overload may be assumed 
for precast pipe. The experiments indicate that some of the smooth- 
est pipe is of careful monolithic construction; likewise some of the 
roughest. Thus, while a very smooth surface may be attained, it 
can not be anticipated with the assuredness possible where units 
are precast under the best practice and subject to the conditions 
attainable in a pipe yard. 
For all types of pipe, competent engineering supervision and 
inspection are necessary if the best results are to be obtained. 
Velocity and entry heads should be provided at the intake. Align- 
ment and grades should be true. Changes in either should be made 
with as gentle curves as are feasible. Unless it is excluded above 
the intakes, means should be provided for the removal of debris at 
the foot of steep upward slopes. Means for the removal of air, not 
only at the summits but also near the intake (unless deeply sub- 
merged) should be provided. This is true of small orchard lines as 
well as large siphons. Trash racks, originally placed at the intakes 
of many siphon pipes, have since been removed in many cases 
because the danger from washout, due to the accumulation of trash, 
outweighed the good accomplished. 
For all practical purposes it does not appear necessary to limit the 
higher velocities. The velocity should be at least sufficient to 
prevent the deposition of silt — probably not lower than 3 feet per 
second. 
1 Water Works Handbook, by Flinn, Weston and Bosert, New York, 1916, p. 276. 
2 Jour. West. Soc. Engrs., Vol. XVI, No. 8, 1911, p. 696. Discussion of C. C. Saner. 
