ON THE RESISTANCE OF ROAD VEHICLES TO TRACTION. 347 



The relative value of the various roads are being compared by taking 

 viagrams by means of a special instrument, the viagraph, kindly lent by 

 the inventor, Mr, J. Brown, F.R.S., of Belfast. 



XV. The Work of the Committee fresented at the Meeting. 



After a good deal of delay, due to various causes, the dynamometer 

 was at length completed about August 20. It was then calibrated m the 

 following manner. The car and dynamometer were placed on a smooth 

 horizontal floor with a 40-inch lurry-wheel mounted in the frauie. The 

 car itself was prevented from moving backwards, and a given load was 

 attached to a wire fixed to the wheel at the top and passing over the tyre, 

 so that it hung vertically, thus tending to pull the frame away from the 

 car After a load was applied the apparatus was shaken to prevent it 

 from sticking in any way, the paper meanwhile being uniformly moved 

 until the pencil of the pressure-gauge had moved to its position of equili- 

 brium. This was done with every reading. When the leverage of 8 to 1 

 was being calibrated, increments of 2 lb. were used in almost every case 

 from to a total of 82 lb. For the 4-to-l leverage increments of 14 lb, 

 were taken, after 14 lb. had been reached, the highest reading being 

 168 lb., the 2-to-l leverage had increments of 14 lb., the highest being 

 280 lb., while the 1-to-l leverage had increments of 28 lb. up to a total 

 of 530 lb. Calibration curves were drawn for each leverage, and scales 

 were drawn from these curves. Nine large wall-diagrams have been 

 prepared for the meeting, the first being the calibration curve for the 

 dynamometer when the 4-to-l leverage was in use. 



The tachometer was calibrated by driving it with an electric motor. 

 The exact time of three revolutions of the drum was taken by means of a 

 stop-watch for readings of 10, 15, 20, 25, and 30 miles per hour on the 

 scale (three revolutions of the drum correspond to 315 revolutions of tacho- 

 meter spindle). The mean diameter of the back wheels of the car was 

 taken as 842 mm. when the car was normally loaded and tyres normally 

 inflated ; the diameter of pulley on back axle is 225 mm. ; diameter of 

 tachometer pulley 75 mm. Working out the speed from these data, the 

 calibration curve shown on wall-diagram No. 2 was obtained, thus show- 

 ing that the scale readings are reliable. It might be argued that if the 

 driving wheels of the car slipped on the road, the tachometer is really 

 reading higher than it ought to do ; but as readings are only used for 

 which the velocity is constant very little slip may be expected ; and since 

 it is not impossible for the driving belt of the tachometer to slip, thus giving 

 too low a reading, it is not too much to assume that these errors will 

 Ijalance each other to some extent. 



The method of operating the dynamometer and performing a trial 

 is as follows -.—The castor frame is pushed towards the car, so that the 

 ram is as far out of the cylinder as possible ; the bulb, having been 

 previously filled with water, is squeezed by the hand, so that the water 

 is forced through the cylinder, through the connecting tubing, and out 

 through the opening in the end of the tube of the pressure-gauge. This 

 method of filling ensures the removal of all air. When all the air has 

 been removed the cocks at each end of the system are closed. The stops 

 have now to be adjusted so that the maximum pressure of the water 

 cannot exceed 100 lb. per square inch, or else the gauge might be 

 destroyed. After the adjustments are satisfactory a run is made to 



