12 
PRACTICAL PART 
stem to prevent cattle from rubbing it. The 
organisation of the bark is easily mechanically 
injured and destroyed by any bruise, or by 
cattle rubbing against it. It is the common 
error to believe that the bark is chemically poi¬ 
soned by the oil from the skin of cattle. 
It may be found necessary to stay the sway of 
the tree, while in movement, by ropes from the 
stem to the shafts. 
In going down steep hills the machine may be 
held back by a horse ridden behind, and hooked 
on by the South American surcingle and single 
trace; or the machine may be very conveniently 
blocked by lashing a bar of wood across the 
shafts, close in front of the wheels, and taking 
out the screw which fastens the shafts to the 
axle. The horse will then bear back against the 
wheels instead of against the axle, and the fric¬ 
tion against the wheels will increase or decrease 
directly as the downward impetus. 
This application of a common mode of block¬ 
ing wheels has the advantage of being entirely 
self-acting. It needs neither the attention nor 
personal service of the driver; nor is it left to 
his judgment at what time, or how much, or 
how little, to put on or to take off. Without 
any stoppage or trouble, friction is acquired at 
