818 REPORT— 1902. 



the pressure increases ; consequently wood pavements are not suitable 

 for heavy traffic' 



The law for the ratio of rolling friction to the pressure, enunciated 

 by Coulomb and others, is not a general or mathematical law, but only 

 approximately true for certain cases in which it is found useful to apply it. 



Coulomb'' s Law, 



r 

 Values for A, the constant, are as follows : — 



1. For oak wheels running on poplar boards, the grain of the 



oak being perpendicular and that of the poplar parallel 



to the line of motion A = 0-00087(1 



2. For oak wheels on strips of leather A = 0-001895 



3. For oak wheels on plaster A = 0-000821 



Experiments on the Haulage of Vehicles. 



For the following experiments, General Morin made use of three kinds 

 of apparatus : — 



I. A shaft, to which a number of equal pulleys were attached, so that 

 the width of surface in contact could be varied ; a number of metal discs 

 were attached to the axle to act as a load. 



The force required to produce uniform rotation was found by trial on 

 well -beaten clay, fine sand, slabs of marble, boards, etc. In each case 

 the resistance (K) was calculated, being equal to the motive weight 

 multiplied by the inverse ratio of the diameter of the axle to the diameter 

 of the pulleys. Thus it was found that the resistance varies with the load 

 and other circumstances met with in the experiments. 



II. A shaft carrying a number of discs to vary the weight, and having 

 two wheels formed by a number of pulleys. In this manner the diameter 

 of the wheel and the width of the tyre could be varied at will. 



The axle was attached to a metal frame, to which one or more horses 

 were harnessed. A dynamometer recorded the tractive effort by means 

 of a pencil in contact with a roll of paper which was unrolled, the move- 

 ment of the paper being proportional to that of the wheel, and the 

 deflection of the spring to that of the tractive effort. The area of the 

 figure enclosed measured the work. 



III. A dynamometer was interposed between the shaft and the fore- 

 carriage. The tractive force was quite easily transmitted by this method. 

 Most of Morin's experiments were performed in this manner. 



The causes that have an effect on the tractive effort and also tend to 

 destroy the roads are : — 



(i.) The weight or pressure upon the ground, 



(ii.) The diameter of the wheels. 



(iii.) The width of the tyres. 



(iv.) The velocity of the vehicle. 



(v.) The angle of inclination of the pull. 



(vi.) The efficiency of the springs. 



From each experiment Morin obtained a value for F, the trftctire 



