38 BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 
parative cost of power in the various districts. These data indicate 
that where cheap coal or gas is available steam is the most economical 
source of power. 
Return tubular, marine, locomotive-type, and water-tube boilers 
have all been used. For a small plant the locomotive type or the 
simple return tubular boiler is simplest and cheapest. For plants 
of over 200 horsepower water-tube boilers have decided advantages, 
though they may cost somewhat more than the simpler fire-tube 
boilers. The boilers of all large plants should be kept constantly 
insured by some reliable boiler insurance company. 
KIND OF ENGINE OR MOTOR. 
Either vertical or horizontal engines may be used. When possible, 
it is best to have the pump directly connected to the engine. This is 
usually not feasible for a pump smaller than the 24-inch size, because 
such a pump would require too high a speed of the engine. In this 
case the pump must be driven by a belt or a rope. The engine should 
be condensing and, if large, should be compound. A high-speed 
automatic engine is more difficult to operate with inexperienced labor 
than a slow-speed engine, but it has the advantage of being smaller 
and fighter for the same power than the slow-speed engine of the 
Corliss or some similar type. In general, however, the high-speed 
engine is not as economical of steam as the slow-speed type. The en- 
gine should be provided with a governor able to keep it from racing if 
the entire load is suddenly removed. The engine should be adjustable 
for a considerable range of speed, should be able to carry a consider- 
able overload, and should work as economically as possible through 
a wide range of load. As will be explained more fully below, it is 
especially desirable that an engine for this work should be capable 
of very flexible operation. 
If a motor is used to drive the pump, it should be of the constant- 
speed slip-ring type. If the motor is direct-connected to the pump 
or is connected by some means that does not allow the speed ratio 
to be changed, the pump will work satisfactorily at only one head; 
that is, the head suitable to the fixed speed. If the lift be higher, 
the quantity pumped will be much reduced, while if the lift be lower 
the quantity will be much increased, the efficiency lowered, and danger 
of overloading the motor will be incurred. Manufacturers have 
recently placed on the market a centrifugal pump with a modified 
screw-type impeller which requires about the same horsepower for a 
rather wide range of heads. The screw pump previously described 
also has this feature. Both the screw pumps and the centrifugal 
pumps with the modified screw-type impeller have been connected 
to motors with a fixed-speed ratio, such as by the use of gearing, and 
have worked satisfactorily. However, as the head increases the 
