216 LOGGING 
boom which may be lowered when the machine is moved from 
one set-up to another. Some types also are built to operate two 
overhead lines from one spar. The one which uses a tree for 
a head spar and which skids for comparatively short distances 
has a main cable from 1 to 1| inches in diameter suspended 
between two supports known as the "head spar" and the "tail 
spar." These usually are from 600 to 750 feet apart, although 
spans of 5200 feet have been used in mountainous country. 
Head spar trees are located along the railroad at intervals of ap- 
proximately 1000 feet. They are selected by the foreman before 
felling operations begin, must be straight and sound, and should 
have a minimum diameter of 18 inches at 60 feet above ground. 
In order to make the spar more stable the trees are topped before 
the rigging is placed. 
A heavy steel spar mounted on the skidder car now often 
replaces the head spar tree required by the earlier type and 
is so constructed that it can be lowered to facilitate moving the 
skidder from one set-up to another. This spar, for relatively 
short-distance skidding, is about 75 feet high and is so adjusted 
that it can be lowered upon the end of the loading boom when 
the machine is moved from one set-up to another.^ The machines 
used in skidding for distances of several thousand feet usually 
are of a different type. The booms are either cylindrical or 
square in cross section and the base rests upon the framework 
of the skidder. When the machine is moved the spar is lowered 
upon an empty car placed in front. The placement of the blocks, 
the guying of the steel spar and the adjustment of the main 
cable after it has been placed on the ground ready for connecting 
up, requires from 15 to 30 minutes, while a day is needed to take 
down the tackle, move the skidder, and adjust the blocks on a 
head spar tree. The great weight of the steel spar skidder makes 
it unsuitable for use on a light or poorly constructed logging 
railroad. 
Tail trees are selected before felling begins, and should be 
from 150 to 250 feet apart and at least 18 inches in diameter at 
30 feet above ground. 
One end of the main cable is passed around the tail tree at a 
height of 25 or 30 feet and is then carried to a stump or tree in 
the rear to which it is made fast. The tail tree is braced with 
1 See Fig. 62. 
