32 TRENCHING AND SUBSOILING 



them. To shift or displace the pulley at each furrow, the 

 spindle is taken out and put in the next hole, and so 

 on. The beams are kept in the ground by means of strong 

 iron posts, and the device has only to be moved every 

 10 feet. 



This method of fixing the pulley is not without draw- 

 backs, and in pebbly, light, or boggy ground, the pulley 

 rapidly drags the beams out. It has, as a matter of fact, 

 to resist a traction double that resisted by the gin, as the 

 two parts of the cable passing over the pulley are parallel, 

 and the stress on each part is equal to the effort of traction 

 exerted tangentially to the circumference of the drum. 

 Notwithstanding Beauquesne's anchoring plates, the fixed 

 pulley of his machine (the resistance of which does not 

 require to be double that of the effort of the capstan) is often 

 dislodged. We cannot, therefore, expect greater stability 

 on the part of the fixed pulley of Grue's system. This 

 compound machine, the main advantage of which seems to 

 be the rapidity of execution of work, cannot in reality attain 

 much greater rapidity, as the displacement of the pulley, 



Fig. 12. Durand's Turn-wrest Plough, with Subsoiling Tines. 



