40 BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 
200 revolutions per minute, and hence may be directly connected 
to two separate engines. 
Assume that the difference between the river-surface level outside 
the district and the desired ground-water level within the district 
will vary from a possible maximum of 20 feet during spring floods 
to nothing in the late summer. The machinery must have sufficient 
power to pump against the highest head, although much of the 
time it will run against a lower head. This is a disadvantage, so far 
as the efficiency of the machinery is concerned, because every engine 
runs most efficiently at about its. rated capacity, and if it is either 
overloaded or underloaded its efficiency is reduced. The actual head 
against which the pumps must force the water, sometimes called 
the dynamic head or the hydraulic head, is always greater than the 
difference in water level as described above, which is called the 
static head. This increase is due to the friction of the water in the 
suction and discharge pipes and the so-called velocity head — that is, 
the head necessary to start the water into motion in the pipes. The 
friction in pipes carrying water at a high velocity is greatly affected 
by the kind of pipe used and the smoothness of the interior surface, 
and hence can not be predicted with certainty. For a velocity of 8 
feet a second the friction in 200 feet of 48-inch pipe would probably 
amount to 2 feet of head and in a 24-inch pipe to at least 6 feet of 
head. A bend of 90° adds probably a foot to the friction head. The 
velocity head amounts to about 1 foot, and if the water passes around 
a sharp edge in entering the suction pipe .there is probably a loss of 
another foot of head. To be safe, then, we should assume the addi- 
tion of at least 6 feet to the static head to give the head on the pump. 
Another small amount might be added to the lift on account of the 
lowering of the water level in the supply ditch when the pumps are 
being operated. But it is probably better to ignore this last addi- 
tion and assume that during the short time when the river is at 
maximum flood stage the water level in the district will not be 
pumped down quite to its normal level rather than to provide extra 
capacity for the very short and infrequent periods of maximum 
river floods. 
With a total head of 26 feet the hydraulic horsepower is calculated 
by multiplying together the lift (26 feet), the number of cubic feet 
of water per second (105), and the weight of 1 cubic foot of water 
(62.5 pounds), and dividing by 550. In this case the result is 310. 
The power required of the engine will be greater than this amount, 
depending upon the efficiency of the inimps, which for these condi- 
tions should be at least 60 or 70 per cent. For this case probably 
manufacturers would be willing to guarantee an efficiency under 
test of 70 per cent. Accordingly the maximum brake horsepower 
required of the engines would be 440. Engines suitable for this 
