LAND DRAINAGE BY MEANS OE PUMPS. 37 
capacity by 50 per cent, but at this overload the efficiency of the 
pump is but a small part of its best figure. 
Rotary displacement pumps and plunger pumps have occasionally 
been used for pumping drainage water, but they are not adapted to 
this use. They may have a very high mechanical efficiency, but 
their cost more than offsets this advantage. Scoop wheels have been 
extensively used in England and Holland, and to some extent in this 
country in Louisiana. They may be made to have a higher efficiency 
than centrifugal pumps, especially for very low lifts, but they are not 
adapted to locations where the head is subject to wide fluctuations. 
A full description of the construction and operation of these scoop 
wheels has been given by W. W. Wheeler. 1 Another publication 2 
describes a recent large and sucessful installation in Holland, and 
gives the results of tests. 
Recently large screw pumps have been installed for the pumping 
of drainage water from districts in Louisiana. One district of about 
9,000 acres has installed and is operating two such pumps, each 
having a diameter of discharge pipe of 78 inches. The city of New 
Orleans recently contracted for and is now installing 12 such pumps 
for the disposal of drainage water; these have diameters of discharge 
of 12 feet. The impeller in the screw pump is a segment of a screw 
and is somewhat similar to a boat propeller, except that it has many 
more blades. On the discharge side diffusion vanes take the water 
from the blades of the impeller and change its partly spiral motion 
to one which is parallel with the axis of the discharge pipe. It is 
expected that such a pump will operate more satisfactorily under 
changing heads than will a centrifugal pump, and for very large 
installations it is cheaper than a centrifugal pump of equal capacity. 
SOURCES OF POWER. 
Steam, gas, and electricity are all used as sources of power for 
pumping plants. In Illinois soft coal is abundant and cheap, though 
in some places crude oil or natural gas may be found more available. 
A suction gas producer with a gas engine has been tried, but such a 
plant requires hard coal or coke for fuel, which greatly increases the 
expense. Furthermore, such a type of plant is not adapted to inter- 
mittent working. Electricity is by far the most convenient source 
of power where it is available, but in general it will be considerably 
more expensive than using coal to produce steam. Its convenience 
will perhaps offset its cost, especially for a small plant. In the latter 
part of this bulletin, under the heading " Amount and cost of pump- 
ing," data from a number of districts are given which show the com- 
1 The Drainage of Fens and Low Lands by Gravitation and Steam Power. London and New York, 
1888. 
2 Engineering News, 63 (1910), No. 20, p. 581. 
