MEASUREMENTS 287 



similar vehicles. It is necessary to determine the effort of the sup- 

 port and that from which the progression results. For the former 

 it will be sufficient to replace the muscles of the arm by a rope tied 



to the handles at its two ex- 

 tremities and hooked in the 

 middle on to an ordinary 

 balance ( 205). For the 

 effort of progression the in- 

 clination of the handles 

 during transport must be 



r*. 188, \^ / y\ taken into account. In 



fact, the effort exerted on 

 a handle, in progression, 

 Effort of progression of a cabrouet. acts by virtue of its hori- 

 zontal component, OF', 



and it is known ( 3) that OF'' varies as the cosine of the angle a, 

 But the cosine varies inversely with the angle a, that is to say, 

 the inclination. The necessary value OF' to overcome the 

 friction of the wheel on its axis and on the ground will therefore 

 depend, to a large extent, on the inclination (fig. 188). 



Again, the friction varies according to whether the centre of 

 gravity is nearer to or further from the axis of the wh<el, and 

 whether part of the weight has a greater or lesser effect on the 

 workman's arms. 



Thus the measurement of the effort must be made under per- 

 fectly definite conditions. Prof. Imbert was content to measure 

 the effort of progression when the centre of gravity of the load 

 was vertically over the axis of the wheels : he joined the handles 

 by a rope whose centre was attached by another rope to a scale 

 pan after having passed over a pulley. Weights were put into 

 the scale until the progression of the truck started (no notice was 

 taken of the effort of the start, which is always a little more). 



The real measurement of the muscular effort shculd be obtained 

 by introducing a graduated dynamometer between the handle 

 and the hand and pushing until the inscribing style traces 

 ordinates equal to the ordinates resulting frcm the progression. 



216. The calibration is very simple in the case of the pruning 

 shears (fig. 189). The handle N'L' is fixed into the jaws of a 

 vice, so that the part SN is horizontal, and the spring of the 

 pruning shears (shewn dotted) is replaced by a rigid piece. The 

 calibration can then be effected by hanging weights from the 

 middle of SN. If the weights are -plotted, as abscissae, a curve 

 like that of fig. 190 would be obtained. The other tools are cali- 

 brated on the same principle. 



