126 BULLETIN 111, TT. S. DEPARTMENT OF AGRICULTURE. 
The main engine, or what is known as the skidding engine, has 
12 by 12 inch cylinders and three drums. The back drum, the one 
next* to the boilers, is known as the skidding line drum. This drum 
hauls the log in. The middle drum is known as the return or trip 
line drum. This is used not only to haul the carriage back to the 
woods but to hold the carriage in proper position when the log is being 
yarded under the carriage, also to regulate the speed of travel of the 
load when gravity carries it forward. The return line drum is pro- 
vided with an interlocking and reversing device, which permits the 
interlocking of the skidder and receding line drum after the ends of 
the logs have been elevated sufficiently to clear obstructions. By 
means of this device the return line is paid out automatically as the 
skidding line is drawn in. The third drum from the boiler is the 
slack-pulling drum, which enables the engineer to pay out from the 
carriage the length of skidding line that is necessary to reach the logs 
that lie to one side or the other of the overhead cable. The slack- 
en «■*£_ 
Fig. 40. — Skidding carriage, Lidgerwood overhead system. 
pulling drum has a friction device set with a tension of about 500 
pounds, so that the line will not hang slack. 
The loading engine has double cylinders and four drums, two 
friction and two clutch drums. The friction drums are used in 
loading, but only one of them at a time, The two make it possible 
to shift the loading line from one side to the other, so that it is unnec- 
essary for the loading engineer to look over the machine when loading 
is in progress. The two clutch drums are used for raising the main 
cable, rigging, etc. 
The machine, as a rule, is mounted on a steel car, which is 22 by 8 
feet and is equipped with a jack at each corner and swivel trucks. 
YVith this equipment it is possible to jack up the car, turn the trucks, 
lay short pieces of steel rail under the car wheels at right angles to 
the track, and move the car from the track. 
The skidder may be mounted on a sled ; in fact, some operators find 
it suits their purpose best to mount it this way. Xo other method 
could be used where it is necessary to place the skidder at some point 
other than along the track. 
