30 BULLETIN 300, U. S. DEPARTMENT OF AGRICULTURE. 
There is also the cable-swing excavator in which the swinging is 
done by means of two cables having dead ends on the perimeter of 
the turntable. These cables run to drums on the upper platform 
which are operated by the swinging engines. This mode of swing is 
not as popular as the rack-and-pinion swing, due to the fact that 
although the machine can revolve nearly a full circle, it must return 
in the direction from which it has revolved. The cable-swing machine 
is the lighter. . 
In the nonrotating drag-line machine the engine platform is fixed; 
the boom is pivoted at its lower end and is the only part of the ma- 
chine which swings. This type is illustrated in Plate VII, Figure 2. 
The crew necessary to operate a drag-line excavator consists of 
two men, an operator and a fireman on a steam machine, an operator 
and an oiler on a gasoline or electrically driven machine. In addition, 
two or more trackmen are required, except in the case of the walking 
and caterpillar types. 
Where the ground is uneven or cut up with old channels and surface 
ditches it is necessary for all excavators not of the rotary type to block 
or bridge across the depressions, laying heavy timbers on whick to 
move the machine. When a machine weighs 25 tons or more the 
expense of providing a solid foundation is an important item. In the 
rotary type of excavator the machine can be revolved to build its own 
foundation of earth. 
THE ROTARY TYPE. 
METHODS OF PROPELLING. 
There are three kinds of mountings used with revolving drag-line 
excavators. The one in general use is the skid-and-roller mounting. 
The machine travels on a track of plank laid on the ground and is 
moved by partly filling the bucket and using it as an anchor upon 
which to pull. Plate IV, Figure 1, shows a machine with skid-and- 
roller mounting transferring a section of its track ahead so that it can 
move up. The skid-and-roller mounting can be used under all ma- 
chines except those weighing over 80 tons. Black-gum rollers are 
ordinarily used, being cheaper and easier to obtain than hard maple. 
Heavy machines are very hard on wooden rollers; consequently trucks 
running on tracks are used for the large sizes. Where trouble is 
expected from crushing and splitting of wooden rollers, 6-inch steam 
pipe may be used instead. In Figure 1 is shown a sectional track for 
a drag-line excavator with skid-and-roller mounting. This figure, 
taken from Engineering and Contracting, volume 46 (1916), page 
158, shows the arrangement of the track units. Each unit is 24 feet 
long, 10 being used for the machine in question, 5 under each side. 
It will be noted from the figure that the ends of the top timbers are 
