62 



THE ANIMAL AS A MACHINE. 



The resistance of all vehicles on common roads and 

 streets is principally resistance to rolling, their axle- 

 friction being comparatively small. The work of haul- 

 ing is, then, 



■ U=Fs= fWs = fWvt (3) 



The draught of vehicles loaded in any stated manner 

 may be made comparatively easy or difficult by proper 

 or improper methods of attachment of the animal to 

 the vehicle.* 



The general principles to be observed are the fol- 

 lowing : 



(1) In hauled loaded vehicles, the line of traction 

 should be made such as to make the hauling power 

 dependent upon adhesion between the animal and the 

 ground equal to the resistance of the vehicle, with 

 some margin of insurance against occasional slipping. 



(2) The heavier the load, if in excess of the hauling 

 power due the animal's weight, the more should that 

 weight be reinforced by so adjusting the line of trac- 

 tion that it may have a vertical component tending to 

 raise the load and increase the holding and hauling 

 power of the feet of the animal. 



(3) With loads lighter than those demanding the 

 total adhesion due the weight of the animal, the line 

 of traction should be so located that the haul, and, if 

 possible, the load itself, may take off a part of the 

 animal's weight. 



Thus, with a two-wheeled vehicle the load may usu- 

 ally be so distributed as either to be carried, in part. 



* Mr. T. H. Brigg has made a study of this point, with interesting 

 results. Trans. Am. Soc. M. E , 1893. 



