LAND DEAINAGE BY MEANS OF PUMPS. 41 
work should be able to carry continuously an overload of 25 per 
cent without an increase of speed and without a great loss in steam 
economy; hence, if 440 horsepower puts this overload on the engines, 
their normal rating would be 350 brake horsepower. They should 
develop sufficient power at the speed at which the pump has the 
best efficiency for a static lift of 10 feet or a dynamic lift of 16 feet, 
in this case about 270 horsepower. When the pumps operate at the 
maximum lift, their speed should be increased in order to maintain 
the discharge and the efficiency of the pumps, and the engines should 
be adjustable and so constructed as to run safely at the higher speed. 
Since increasing the speed of itself increases the power developed 
by the engines, this gives a margin of safety in the capacity of the 
engines. If two separate engines and pumps are used, each engine 
should have a brake horsepower of 175. Its indicated horsepower 
will be from 10 to 20 per cent greater. An engine should run fairly 
economically down to three-fourths of its rating. These units, then, 
would be economical of fuel down to a static lift of 10 feet. Records 
of plant operation on districts on the Illinois River show that this 
is quite near the average pumping head. As the above plant is 
designed, when the lift falls below 5 or 6 feet the steam used by the 
engine will be much increased in proportion to the useful work actually 
performed in lifting the water. This disadvantage is not very im- 
portant for plants on the Illinois River, but for districts on the Missis- 
sippi River, at least for those above Burlington, Iowa, where records 
were available, it is of very great importance, as the mean lift is quite 
often less than 5 feet. Records from the Louisa-Des Moines Drainage 
District No. 4 show the following average static lifts: 1910, 4.20 feet; 
1911, 2.82 feet; 1912, 5.20 feet; 1913, 5.74 feet. As will be shown 
later in this bulletin, under the heading "Amount and cost of pump- 
ing," the cost per unit of water lifted 1 foot was greatly increased 
in 1911, when the lift was very low. Where the average lift is as 
low as 5 feet and the maximum static lift is as great as 20 feet, it 
would be very difficult to install an engine that would work with 
economy at the low head and deliver the full capacity of the pump 
at the highest head. In the above example, for service on the 
Mississippi River it would be better to install a smaller engine than 
one with a nominal rating of 350 brake horsepower and to work it 
at a heavier overload than 25 per cent at the maximum head. Such 
an engine would then work at about three-fourths of its nominal 
rating at a static lift of 6 feet and would not lose much in economy 
of steam consumption. 
In case it were desired to drive the pump by a motor, the calcula- 
tions for the maximum required horsepower would not differ from 
those in the foregoing example. However, as the motor is not capable 
of an overload its nominal rated horsepower must be large enough to 
