228 
LOGGING 
travels. One end of the line is fastened to one of the drums and 
then passes up to and through a block on the spar tree, thence 
to and through a block at the end of the run, thence back to and 
through another block on the head spar tree and then down to 
the other sky-line drum (Fig. 71A). The tail block is fastened 
to the tail spar tree as shown in Fig. 71 A. When used as a yard- 
ing machine on uphill or downhill pulls or on level ground the 
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FiG. 72. — A Duplex Aerial Yarder logging a Steep Slope from a Setting 
in the Valle^'. 
haul-back line block is fastened to a stump near the logs to be 
skidded, the line serving as a slack-pulling device when the sky- 
line is lowered. When yarding in deep canyons, the haul-back 
block is placed near the tail spar and the trolle}^ is run out to 
the desired spot, the sky-line lowered and the chokers attached 
to the trolley. The general scheme of arrangement of cables 
for side-hill logging is shown in Fig. 72. 
This system may be used for spans from 700 to 5000 feet. 
The longer distances may be used when crossing canyons where 
a suitable cable deflection may be secured. The output per day 
may be as high as 100,000 board feet. 
