relative to 'Experiments on Wheel-carriages. 85 



overcome the friction of these axletrees, when compared with 

 the power necessarv to draw the whole load upon a common 

 road, the effort of the horses must be stated as being; equal to 

 some given weight acting with a given velocity. Each horse 

 drawing a mail-coach at the rate of seven English miles per hour, 

 upon an ordinary road, may be considered as exerting a power 

 equal to one hundred pounds. Sometimes a horse exerts nearly 

 three times this power, and sometimes, on good roads, mucK 

 less ; but, perhaps, one hundred pounds may be taken as an 

 average *. 



We may fairly state, that as eleven pounds were necessary to 

 overcome the friction of the arms of the axletree, when the car- 

 riage was loaded with 890 pounds, if the arms of the axletree 

 were loaded as in a mail-coach, with something more than four 

 thousand pounds, it would require fifty pounds to overcome the 

 friction of the axles ; for the power of four horses drawing a 

 mail-coach on ordinary level roads, may be estimated at four 

 hundred pounds, one-eighth of which, viz. fifty pounds, may be 

 considered as the resistance occasioned by the friction of the 

 axletrees. 



Creeping. 



In most carriages the arms of the axletrees are bent a little 

 downwards, so tiiat the wheels are four or five inches further 

 asunder above than below. Besides this, in some carriages the 

 arms of the axletrees are bent forwards, so that the wheels are 

 nearer together before than behind. 



To determine the effect of this construction, which by work- 

 men is called the creep of the wheel, the following experiment 

 was tried : 



A two-wheeled carriage, with the axletree bent downwards in 

 the common manner, was drawn on iron by 1-1 lbs.; on wood by 

 14^. When the wheels were 4j inches nearer together at the 

 front than behind, it required to draw the same carriage on 

 wood 26 lbs. and on iron 20 lbs. Hence it appears that it is of 

 great consequence in the formation of a carriage with bent axle- 

 trees, to secure the axletrees in their proper situation, and to 



will pinch the upper part of tlio sirm of the axletree near its shoulder. 

 Douhk-dislitd wlieels arc not liahle to this defect. Tlic auiount of stiper- 

 riuous frii:tiou that arises from the circuinstanee liere alhiled to, may he 

 ealcuh'itcd hy any mechanic, in any };ivei> position of liic wheel, arising 

 iron) the ohliqnity of the road, or from its sinking into hnjcs. 



' A variety uf opinions upon this suhjcct are luld hy (lillerent authors, 

 and hy difTercnt practical mechanics. As to the coiichisions which 1 mean 

 to draw, it is of little consequence whether the force of a horse in driiwini; 

 H carriage be estimated higher or lo«cr liian what I have assumed. 



F 3 prevent 



