EXCAVATING MACHINERY USED IN LAND DRAINAGE. 54 
the reading on the gauge is greater when pumping solid material 
than when pumping ‘water only. The reading on the pressure 
gauge varies with the length of the discharge pipe used and with 
the elevation to which the material is pumped. An experienced 
operator, however, can tell when the operations are being carried on 
properly. The reading in inches on the vacuum gauge can be 
reduced to the equivalent head in feet by multiplying the number 
of inches by 1.13. The reading on the pressure gauge, in pounds 
per square inch, can be reduced to the equivalent head in feet by 
multiplying the gauge reading in pounds by 2.30. The practical 
maximum discharge pressure is from 45 to 55 pounds, depending 
somewhat upon the size of the pump. For extra high heads, relay 
or booster pumps are used; that is, the first pump delivers the ma- 
terial through a certain length of discharge pipe into the suction 
line of the relay pump. An auxiliary pump is often used to dis- 
charge water continuously into the shell of the dredging pump to 
aid in moving the material pumped. 
The suction head is the distance from the surface of the water to 
the center of the pump plus pipe friction and losses of head through 
the rotary cutter and at the entrance of the suction pipe. The total 
suction head should not ordinarily exceed 25 feet. a condition easily 
met by the hydraulic dredge. The discharge head is the difference 
in level between the pump shaft and the point of discharge, plus 
the friction in the discharge pipe. 
For priming the pump a small centrifugal pump is commonly 
used. The method is to raise the suction pipe until its end is higher 
than the dredging pump and prime until priming water discharges 
from the suction pipe; the suction pipe is then dropped into the 
water and the dredge pump started. 
SELECTION OF EQUIPMENT. 
In order to select the proper equipment several ruling factors 
must be considered. The character of the material to be excavated, 
the maximum depth of water from which the material is taken, the 
maximum elevation at which the material is to be deposited, and the 
maximum and minimum length of discharge pipe to be used, are 
conditions which must first be determined. Likewise the quantity 
of material to be excavated per shift must be decided upon, as well 
as the type of power equipment and method of drive. 
The percentage of solids moved depends upon the character of 
‘material pumped and the velocity in the discharge pipe. In mud 
or silt 20 per cent or more solid material may be handled with the 
- water; in sand and gravel probably not more than 10 per cent. Light 
