432 THE HUMAN MOTOR 



Hence it is only for loads exceeding this figure that the wheel- 

 barrow offers any advantage, it is interesting to compare this 

 result with those given in para. 287 above, where it was shown 

 that the maximum economical load which can be carried directly, 

 by a labourer, is also 21 kg. 



A person using a wheelbarrow seldom employs muscular 

 effort alone. The bent attitude generally adopted in wheeling 

 a barrow shows that the weight of the body is also utilised. 

 Hence a heavy man can do more work than a light man. 



Imbert measured the tractive effort exerted in wheeling a 

 loaded barrow along an ordinary gravel path (see para. 211 for 

 the method of experiment). Under these conditions, the results 

 obtained may be represented by the following formula : 



F. (tractive force) = -043 X P (load). 



With a total load of 110 kg., wheeled on ground soaked by 

 rain and very heavy, the tractive force rose to more than three 

 times the above figure 



F' = -132 x P. 



Leaving out of account, for the moment, the question of static 

 forces, we can calculate that the work done by a man weighing 

 70 kg. in wheeling a barrow weighing 29 kg. loaded with 20 kg., 

 a distance of 1 kilometre is : 



(20 + 29) -043 X 1,000 + 7Q X = M82kgm 



If, in a working day, the man covers 30 kilometres, the work 

 done per diem would be 194,460 kilogrammetres. 



We ought to add to the weight of the labourer a weight numeri- 

 cally equivalent to the downward pressure on the arms. For 

 the same distance covered as above, but with a load of 60 kg, 

 the total daily work would be : 



(29+60) -043 X 30,000 + ( 70 + 2( ^ 3Q ' QQ( > = 332,310 kgm. (approx.) 



The above assumes that the weight sustained by the arms is 

 42% of the total load, i.e., 36 kg. 



Under similar conditions the transport of loads by a wheel- 

 barrow calls for more effort than by a stone truck (cabrouet), and, 

 as Imbert observed, entails a greater increase in the rate of re- 

 spiration (40%) and an increased, and more persistent, fatigue, 

 especially in the arms. 



The above outputs of work are in excess of those obtained from 

 the average labourer, whose working day appears to be generally 

 only 8J hours. 



329. Early Observations. Coulomb measured, by means of 

 a spring balance, the effort exerted in wheeling a barrow weighing 



