WHEEL. 



f: 



it is evident that when we augment their diameter we add 

 greatly to their weight ; and by thus increafing the load, 

 we fenfibly diminifh their power. 



From thefe remarks, we fee the fuperiority of great wheels 

 to fmall ones, and the particular circumftances which fug- 

 eft the propriety of making the wheels of carriages lefs than 

 .our feet and a half radius. But even this fize is too great ; 

 and it may be fafely alTerted that they fhould never exceed 

 fix feet in diameter, nor ever be lefs than three feet and a 



half. 



6. Carriages with four wheels, as waggons or coaches, are 

 much more advantageous than carriages with two wheels, as 

 carts and chaifes ; for in applying horfes to a carriage with 

 two wheels, it is plain that the tiller carries part of the 

 weight, in whatfoever manner it be kept in equilibrio upon 

 the axle. In going down a hill, the weight bears upon the 

 Tiorfe ; and in going up a hill, the weight falls the other 

 way, and lifts the horfe, by which means part of his force is 

 loft. Befides, as the wheels fink into the holes in the road, 

 fometimes on one fide, fometimes on the other, the fhafts 

 Itrike againll the tiller's flanks, which is the deftrudlion of 

 many horfes. Add to this, that when one of the wheels 

 finks into a hole or rut, half the weight will fall that way, 

 whereby the carriage will be in danger of being overturned. 



7. It would be much more advantageous to make the four 

 wheels of a coach or waggon large, and nearly of a height, 

 than to make the fore-wheels of only half the diameter of 

 the hind-wheels, as is ufual in many places. The fore -wheels 

 of carriages have commonly been made of a lefs fize than 

 the hind ones, both on account of turning (hort, and to 

 avoid cutting the braces. Crane-necks have alfo been in- 

 vented for turning yet (liorter, and the fore-wheels have 

 been lowered, fo as to go quite under the bend of the crane- 

 neck. See an account of an ingenious contrivance for this 

 purpofe, under Perch. 



Some carriers and coachmen have, indeed, abfurdly al- 

 leged, that when the fore-wheels are much lower than the 

 hind ones, they ferve to pulh them on. However, many 

 difadvantages attend this conftruftion. A confiderable 

 force is loft that would be effeftual, if they were large : the 

 carriage would go much more eafily, if the fore-wheels 

 were as high as the hind-ones; and the higher the better, be- 

 caufe their motion would be fo much the flower on their 

 axles, and confequently the friftion proportionably dimi- 

 niftied. The jolting and uneafy motion occafioned by low 

 wheels, has induced perfons to contrive fprings, in order to 

 prevent it. But nothing can be more inconfiftent, even with 

 this end, than the common method of fixing the braces to 

 the bottom of the body of a carriage. In confequence of 

 this practice, the centre of gravity of the fufpended body 

 is fo high above the centre of its motion, that it is liable to 

 be continually agitated by the jolting of the carriage, and 

 its danger of overturning increafed : whereas if, inftead of 

 praftifing this method, the body were fufpended as near as 

 pofQble to its centre of gravity, the agitation of the car- 

 riage, as well as its danger of overturning, would be in a 

 great meafure avoided. 



The effeft of the fufpenfion of a carriage on fprings is to 

 equalize its motion, by caufing every change to be more 

 gradually communicated to it, by means of the flexibility of 

 the fprings, and by confuming a certain portion of every 

 fudden impulfe in generating a degree of rotatory motion. 

 This rotatory motion depends on the oblique pofition of the 

 ftraps fufpending the carriage, which prevents its fwinging 

 m a parallel direftion ; fuch a vibration as would take place 

 if the ftraps were parallel, would be too extenfive, unlefs 

 they were ve»y (hort, and then the motion would be fome- 



what rougher. The obliquity of the ftraps tends alfo in 

 fome meafure to retain the carriage in a horizontal pofition ; 

 for if they were parallel, both being vertical, the lower one 

 would have to fupport the greater portion of the weight, 

 at leaft according to the common mode of fixing them to the 

 bottom of the carriage ; the fpring, therefore, being flexi- 

 ble, it would be ftill further depreffed. But when the ftraps 

 are oblique, the upper one aflumes always the more vertical 

 pofition, and confequently bears more of the load ; for when 

 a body of any kind is fupported by two oblique forces, 

 their horizontal thrufts muft be equal, otherwife the body 

 would move laterally ; and in order that the horizontal por- 

 tions of the forces may be equal, the more inchned to the 

 horizon muft be the greater : the upper fpring will, there- 

 fore, be a little depreffed, and the carriage will remain more 

 nearly horizontal than if the fprings were parallel. The 

 reafon for dividing the fprings into feparate plates has 

 already been explained : the beam of the carriage, that 

 unites the wheels, fupplies the ftrength neceflary for form- 

 ing the communication between the axles : if the body of 

 the carriage itfelf were to perform this office, the fprings 

 would require to be fo ftrong that they could have little or 

 no effeA in equalizing the motion, and we ftiould have a 

 waggon inftead of a coach. The eafe with which a carriage 

 moves, depends not only on the elafticity of the fprings, 

 but alfo on the fmall degree of ftability of the equilibrium, 

 of which we may judge in fome meafure, by tracing the 

 path which the centre of gravity muft defcribe, when the 

 carriage fwings. 



There is an inconvenience which attends the ufual method 

 of loading carriages ; for when a carriage is loaded equally 

 heavy on both axles, the fore-axle muft endure as much 

 more friflion, and confequently wear out as much fooner 

 than the hind-axle, as the fore-wheels are lefs than the hind 

 ones. However, the carriers commonly put the heavier part 

 of the load upon the fore-axle of the waggons ; which not 

 only makes the friftion greateft where it ought to be leaft, 

 but alfo prefleth the fore-wheeU deeper into the ground than 

 the hind-wheels, although the fore-wheels, being lefs than 

 the hind ones, are with fo much the greater difficulty drawn 

 out of a hole, or over an obftacle, even fuppofing the weights 

 on their axles were equal ; for the difficulty, with equal 

 weights, will be as the depth of the hole, or height of the 

 obftacle, is to the femi-diameter of the wheel. Moreover, 

 fince a fmall wheel will often fink to the bottom of a hole, 

 in which a great wheel will go but a very little way, the 

 fmall wheels ought to be loaded with lefs weight than the 

 great ones ; and then the heavier part of the load would be 

 lefs jolted upward and downward, and the horfes tired fo 

 much the lefs, as their draught raifed their load to lefs 

 heights. When the waggon-road, indeed, is much up-hill, 

 there may be danger in loading the hind-part much heavier 

 than the fore-part ; for then the weight would over-hang 

 the hind axle, efpecially if the load be high, and endanger 

 tilting up the fore-wheels from the ground. In this cafe, 

 the fafell way would be to load it equally heavy on both 

 axles ; and then as much more of the weight would be 

 thrown upon the hind axle than upon the fore one, as the 

 ground rifes from a level below the carriage. But as this 

 feldom happens, a fmall temporary weight might be laid 

 upon the pole between the horfes, which would overbalance 

 the danger. 



From Mr. Fergufon's obfervations on the centre of 

 gravity, it is evident, that if the axle-tree of a two- 

 wheeled carriage paffes through the centre of gravity of the 

 load, the carriage will be in equilibrio in every pofition in 

 which it can be placed with refpeft to the axle-tree ; and 



