342 LOGGING 
is materially shortened by having two separate sets of drivers 
which permit the use of a heavy rod locomotive on a road having 
curves that are too sharp for the regular type of rod engine of 
the same weight; and it is so constructed that live steam may 
be used in the cylinders of both engines to secure greater power 
to start loads, which increases the hauling power of the loco- 
motive in comparison with that of an ordinary rod engine of the 
same weight, since an engine can keep in motion a greater load 
that it can start. Another feature claimed for this locomotive 
is that the drivers slip less than on other types of rod engines 
because the forward cylinders depend on the rear ones for steam, 
and should the drivers connected to the latter slip, the exhaust 
would fill the feed pipe of the forward cylinders faster than it 
could be relieved and the resultant back pressure on the high- 
pressure pistons would reduce the speed and prevent further slip- 
ping. 
Locomotives of this type, ranging in weight from 81 to 121 
tons, are in use on logging roads in the Pacific Northwest. The 
minimum weight in which they are built is 50 tons. One weigh- 
ing 121 tons is in operation on the Pacific Coast on a road having 
35-degree curves and 8 per cent grades.^ 
Geared Locomotives. — The first geared locomotive was con- 
structed about 1885 by E. E. Shay, a Michigan logger, and this 
locomotive, with modifications and improvements, is in extensive 
use to-day. Several forms of geared locomotives other than 
the Shay are now on the market. 
The objects sought in geared locomotives are to secure a 
maximum amount of tractive force with a minimum total weight, 
a short truck base that will enable the engine to take sharp curves 
with ease, and a form of truck that will adjust itself readily to 
an uneven track. These ends are accomplished by making every 
wheel under the engine and tender a driving wheel; by trans- 
mitting power to the driving wheels through a series of bevel 
gears that bear a ratio to each other of from 2 to 1 or from 2\ 
to 1; and by the use of swivel trucks on which the drivers are 
arranged in pairs and connected, one with another, by means of 
an articulated driving rod. The weight is distributed over a 
long wheel base which permits the use of a smaller rail, fewer 
1 The Timberman, August, 1910, p 63. i 
