TKENCHING AND SUBSOILING. 75 



When the cable drags on the ground it creates a supple- 

 mentary resistance. The weight of the cable is usually 

 8 ounces per foot, the co-efficient of friction being 0-6. 

 Under these conditions the additional effort of traction is 

 0-48 ounce per foot of cable. If we assume a furrow 274 

 yards in length, the additional effort mentioned above at 

 the maximum is: 



231 Ibs. when the plough is 274 yards distant. 

 184-8 219 



92-4 109 



9-24 33 feet 



But in practice it is not necessary to have a greater effort 

 at the beginning of the furrow than at the end, for the cable 

 winding round the drum increases the diameter, and there- 

 fore the leverage as the work proceeds. Finally, we have 

 noticed the enormous traction exerted on the cable diminishes 

 its pressure on the ground, and therefore its friction. 



The ratio between the speed of the end of the pole, and 



the speed of the plough, varies between - and ; the ratio 



usually worked at is 



With the above data it is easy to calculate the effort of 

 traction exerted on the plough. 



Assuming a ratio of speed of 10 to 1. 

 A team of four horses, each capable of developing a trac- 

 tion of 220 Ibs. 



A mechanical yield of the machine of 85 per cent., we 

 find : 



Mean effort of the team : 



220 x 4 x 0-77=677 Ibs. 

 Mean effort exerted on the plough : 



677 x 10 x 0-85=5,674 Ibs. 

 This applies only to simple-effect drums. 



