3) BULLETIN 300, U. S. DEPARTMENT OF AGRICULTURE. 
in muskeg swamps, thus sparing the excavation of several feet of 
the soft surface to reach the underlying clay. 
The caterpillar or apron-traction mounting eliminates the track- 
men necessary with the skid-and-roller mounting. For small ma- 
chines two caterpillars (Pl. IV, Fig. 2) are commonly used. In one 
make of light drag-line excavator a combination of two wheels and 
two caterpillars is used. The heavier machines require four cater- 
pillars. A skillful operator can turn a drag-line excavator mounted 
on caterpillars in its own length. 
Large machines usually are mounted on four 4-wheeled equalizing 
trucks (Pl. V, Fig. 1), though any size may be furnished with this 
mounting if desired. These trucks may all be nonpropelling, in 
which case the machine moves in the same way as 1f mounted on skids 
and rollers, or two of the trucks may be driven by power from the 
main engines of the machine. On the smaller machines one truck 
usually is mounted under each corner of the lower platform, while 
on the larger machines three of the trucks are generally mounted on 
an equalizing beam. This latter method is preferable, as by its use 
the weight of the machine is always evenly distributed, and thus 
the platforms are not subjected to severe stresses. 
There is another type of mounting in which a novel method of 
moving is employed. Attached to the upper platform and extending 
through the machine in a direction at right angles to that of the boom 
is a heavy steel shaft, on each end of which is a wheel segment (PI. 
V, Fig. 2). The shaft also carries a large gear wheel which meshes 
with a pinion on the loading-drum shaft of the main engine. Sus- 
pended from the middle arm of each segment by means of a carrying 
beam and chains is a long shoe, which affords a bearing for the seg- 
ment as it rotates and propels the machine.forward. To move in 
a given direction the excavator is rotated until the boom is pointing 
in the opposite direction; the side shoes are lowered by rotating the 
shaft supporting the wheel segments, and the weight of the machine 
is thrown on to the side shoes; the segments cause the machine to 
rise and move ahead 8 feet. This excavator has an advantage over 
other types of self-propelling machines in that it can move in any 
direction. The machine can be walked at a rate of 25 to 30 feet a 
minute. When digging, the machine rests upon a large circular base. 
The average bearing pressure when working is from 3} to 44 pounds 
per square inch. 
All self-propelling machines do without trackmen when working 
over reasonably stable ground. In soft ground extra bearing sur- 
face may be required to prevent the machine from sinking. On one 
occasion a 38-yard, 70-foot boom, walking, drag-line excavator, work- 
ing in unusually soft ground, required additional bearing surface. 
Eight pontoons were used, each 7 feet by 30 feet, the machine always 
