486 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



root-cutters are now frequently driven by steam. Wherever we have 

 to do with large production it should be, and it is advantageous to 

 substitute for the moving force of oxen or horses the moving force 

 par excellence steam. 



But in no case has the introduction of steam-power been more 

 revolutionary than in the case of ploughing. Of the many systems 

 of ploughing by steam, two only have proved thoroughly successful ; 

 in both of these the traction power is transmitted to the plough 

 through a steel wire rope winding upon a drum. In the one plan the 

 two winding-drums are fixed in a windlass frame, and connected to a 

 stationary steam-engine, which can be worked from one corner of a 

 field ; one end of each rope being made fast to the plough, the imple- 

 ment is drawn backwards and forwards by the drum pulling alter- 

 nately, and the pulley sheaves and anchors at each end of the furrow 

 move forward as the implement proceeds. In the other system each of 



FIG. 3^7. Direct system of steam ploughing. 



the winding-drums is placed under the boiler of a self-moving steam- 

 engine (see Fig. 326), and one engine at each end of the furrow 

 alternately pulls the plough towards it, the other moving forward 

 into position ready for the return of the plough. These two systems 

 are Known as the single engine or roundabout, and the double engine 

 or direct method of steam cultivation. For large farms the double 

 engine or direct system is the best. Land can be ploughed by it at 

 one-half the cost of horse-power. Figs. 326 and 327 show the 

 arrangements of the drum and the action of the two engines when 

 used on the direct system. 



Portable engines have received very varied forms, according to 

 their destination and the ideas of their constructors. 



The boiler is, as in the locomotive, a tubular boiler, composed of a 

 grate A situated behind, and a cylindrical body BB, which incloses 



