THE 1RR1GA 770 A" A GE. 57 



twisting; whereas if so rigid as to be almost torsionless, the pressure 

 of the wheels on the hillocks of the road is augmented by the rigidity 

 of the body, and causes greater draft. The more flexible the spring 

 above each wheel, the less torsional strain to the body, and the lighter 

 the pressure of the wheels in surmounting obstacles on a road-purface. 



This is why between* two vehicles on four wheels, both scaling 

 the same weight the one a wagon, and the other a brougham 

 the wagon would have the least draft, because of its body twisting 

 and adapting itself to the irregularities of the road. It is this aug- 

 mented wheel- pressure that causes draft to be light or heavy. These 

 very terms in common use imply it is the pressure on the road that 

 influences draft. 



A professor of science and applied mechanics was watching a 

 Buckinghamshire (Eng.) wagon-maker at work, and ventured to ask 

 him about the torsion of the wagon body over rough roads. 



The answer was: "I knew a lot of bad language when I was 

 5'oung, but I do not recollect using such a word as that.*' The pro- 

 fessor, smiling at the droll reply, explained that the word meant 

 twisting,- cross-corner straining warping and the resulting counter- 

 strain, and that the one word expressed the action and condition re- 

 currently of the wagon body when strained on uneven roads. The 

 wagon-builder then said: "\Ve arrange for body-twist by not bolting 

 the near hind corner of the wagon tight down to the cross-bed, only 

 long-pinion it with the bolts, with room for two inches play up and 

 down of the body from the bed, when twisted." 



He knew the correct principle without the scientific term for the 

 twisting strain, and its counteraction. 



The inquirer then put the same question to a fashionable coach- 

 maker, and the answer was: "We build our carriages so perfectly that 

 body-torsion is impossible." The professor lifted up one corner 

 of a laudau on elliptic springs, by the hind body-loop, and convinced 

 the coachmaker that a strongly -plated body has torsion under cross- 

 cornered strain: and also convinced himself that although the coach- 

 maker knew what torsion meant, he understood less the laws of ap- 

 plied mechanics than his humble, intuition taught fellow-craftsman, 

 the country wagon-maker. 



The law of torsion in vehicle bodies is always determined by the 

 number of. points of contact under two conditions: the ground-bearing, 

 and the body-bearing on the intercepting carriage and its axle, or 

 axles. This fact will be clear, if we survey the points of contact on 

 the ground, be it level as a die, or rugged as a "parish- job" roadway 

 after a little wear. A vehicle on one wheel would have no body-torsion 

 due to the road, as it would have but one point of contact; nor would 

 it on two, or even three wheels: but on four wheels, a rugged or 

 uneven surface causes both carriage and body-torsion, the one acting 



