MOTIVE POWER AND ROLLING STOCK 305 



single or compound expansion, and the exhaust steam of the 

 rear engine is used in the cyHnders of the forward engine, thus 

 effecting a saving in fuel. 



The advantages of this tj-pe of engine are that the wheel base 

 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 hve 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 

 than 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 engine depends on the rear one for steam, 

 and should the drivers connected to the latter slip, the exhaust 

 would fill the feed pipe of the forward engine faster than it could 

 be relieved and the resulting back pressure on the high-pressure 

 piston would reduce the speed and prevent further slipping. 



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 some 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 



1 The Timberman, August, 1910, p. 63. 



