252 LOGGING 
let pipes. The steam, smoke and gases escape through the screen, 
in which the cinders do not clog because of its conical form. 
The screen can be raised by means of the lever lift (E) when it 
is unnecessary to use an arrester or when firing up the boiler. 
The Boomerang Spark Arrester is used by many loggers on the 
Pacific Coast. This has a hea\'y j-inch mesh round screen 
(A), slightly flaring toward the top, on which is mounted a heavy 
sheet iron cone (B). The latter ends in a boomerang (C) to the 
open end of which a screen conveyor tube (D) is attached. The 
smoke passes out through the screen while the sparks travel 
straight up through the steel cone where they are diverted into 
the boomerang and led into a receptacle by the side of the engine. 
As the sparks do not come in contact with the screen it does not 
become clogged. 
ELECTRICAL DRIVE 
Loggers and the manufacturers of electrical equipment have 
been interested for many years in the development of logging ma- 
chinery driven by electrical power, but only a relatively few 
installations of such equipment have been made. As early as 
1908 an electric road engine was tried out in British Columbia, 
but it did not work satisfactorily because of the inability of the 
motor to vary its speed, and take up the slack in the line on down- 
grade pulls. One of the earliest installations of yarding engines 
with electric drive was made in 1911 and since that time marked 
improvements have been made in such equipment, especially 
since 1918. Although the loggers, in general, have not accepted 
the electric-drive idea in its present stage of development, some 
have made installations which are giving satisfaction. The modern 
electrically-driven donkey engine is a combination yarding engine 
and loader mounted on one sled about 60 feet long. The two-speed 
motor driving the yarding drums has a rated horse-power of 300 
which, by gearing, may be increased to 1200. The loading drums 
are driven by a 75 horse-j^ower, two-speed motor. The skidding 
drum has a capacity of 1800 feet of 1^-inch cable. The gear 
shifts, frictions and whistle are operated by compressed air. 
Power for driving the motors is brought to the vicinity of the 
yarding engine by transmission lines which carry about 13,000 
volts, which is stepped down by a transformer located near the 
machine, to 600 volts. ^ 
