TRACTORS 213 
increase its pull for a very short distance and for a very brief 
period from three to four times its normal for continuous work, 
while the margin between the rated draw-bar H. P. and actual 
maximum for tractors is comparatively insignificant.^ On the 
other hand, a tractor develops its normal power for any length 
of time while animals, especially under adverse conditions, be- 
come fatigued, and then decrease their pull below normal in 
addition to utilizing a part of the working time for rest. 
Large tractors are preferred to small ones, provided they can 
be worked to capacity, because one driver has control over more 
power, and the wage cost per thousand board feet is less. How- 
ever, there are about as many 5-ton as 10- ton tractors used in 
logging and some loggers believe they are more efficient for 
comparatively short distances. The 2-ton crawler tractors, 
though sometimes used in the forest, are too small for most kinds 
of logging work. 
BIBLIOGRAPHICAL NOTE TO CHAPTER XIII 
BrighaM; E. J.: Tractor Logging. The Timberman, Oct. 1920, pp. 86 
and 87. 
GiRARD, James W. : Tractor and Horse Skidding in Inland Empire. The 
Timberman, Nov., 1922, pp. 66, 68, 70 and 72. 
Klobucher, Frank J.: Tractor Skidding Studies in the Inland Empire. 
The Timberman, July, 1922, pp. 114, 116, and 118. 
1 A test of tractors at the University of Nebraska showed that some of the 
best crawler tractors may be overloaded 30 per cent; in actual work, however, 
crawler tractors will not develop much more than their rated power, and in 
any case this margin is very small as compared to a possible 300 per cent 
increase of puU by a horse. 
