WHEEL. 



wheels would be ufeful only in overcoming fridion ; but as 

 they are drawn along roads covered with loofe ftones, indented 

 with cavities, they are farther ufeful in ferving to deprejs, 

 or raife the carriage over the one, and in raifing it out of the 

 other. 



4. The axlee of the wlieels ought to be perfeftly ftraight, 

 and at right angles to the {hafts, or to the pole. When the 

 axles are ftraight, the rims of the wheels will be parallel to 

 each other, and then they will move the eafieft, becaufe they 

 will be at liberty to go on ftraight forwards. But in the 



The wheels of all carriages ought to be exaftly round ; ufual way of pradice the axles are bent downwards at their 



and the fellies fhould be at right angles to the naves, accord- ends ; which brings the hdes of the wheels next the ground 



ing to the inchnation of the fpokes, i. e. the plane of the nearer to one another than their higher fides are ; and this 



curvature of the wheel fhould cut the nave at right ang es, not only makes the wheels to drag fideways as they go 



tiiouffh it need not pafs through the place where the fpokes along, and gives the load a much greater power of crufhing 



• r J • . ..u ,„o them than when thev are parallel to each other, but alio en- 



are inferted into the nave. '. f . r 1 f 11. 



The fpokes according to Mr. Fergufon and moft dangers the overturning of the carriage when any wheel faUs 

 other writers on m'echanics, Hiould be incHned to the naves, into a hole or^rut, o-- when the carriage goes on a road 

 fo that the wheels may be diftiing or concave. If, indeed, 

 the wheels were always to go upon fmooth and level ground, 

 it would be beft to make the fpokes perpendicular to the 

 naves, or at right angles with the axles ; becaufe they 

 would then bear the weight of the load perpendicularly, 

 which is the ftrongeft way for the wood. But becaufe the 

 ground is generally uneven, one wheel often falls into a 

 cavity or rut, when the other doe? not, and then it bears 

 much more of the weight than the other does ; in which 

 cafe difliing wheels are beft, becaufe the fpokes become 

 perpendicular in the rut, and therefore have the *" 



eateft 



which has one fide lower than the other, as along the fide of 

 a hill. Thus, in t'le hind view of a waggon or cart, let 

 A E and B F {Plate XL. Jig. 9. Mechanics) be the great 

 wheels parallel to each other, on their ftraight axle K, and 

 H C I the carriage loaded with heavy goods from C to G. 

 Then as the carriage goes on in the obhque read A a B, the 

 centre of gravity of the whole machine and load will be at 

 C ; and the line of direftion CdT) falling within the wheel 

 B F, the carriage will not overfet. But if the wheels be 

 inclined to each other at the ground, as A E and B F are 

 {Jig. 10.), and the machine be loaded as before from C to 



ftrenffth when the obhquity of the road throws moft of its G, the Hne of direaion C ^D falls without the wheel B F, 

 weight upon them ; whilft thofe on the high ground have and the whole machine tumbles over. When it is loaded 



When it is 

 with heavy goods which lie low, it may travel fafely upon 

 an oblique road, fo long as the centre of gravity is at C 

 [Jig.g.), and the line of direftion C(/D falls within the 

 wheels ; but if it be loaded high with lighter goods from C 

 to L (fg. II-), the centre of gravity is raifed from C to 

 K, which ftiews the hne of direftion K k without the loweft 



lefs'vveight to bear, and therefore need not be at their full 

 ftrength. Befides, by this form of the wheels, the bafe of 

 the carriage is extended, and it is thus prevented from being 

 eafily overturned, and the feUies are hindered from rubbing 

 againft the load or the fides of the cart. Dr. Brewfter, how- 

 ever, is of opinion, that the difadvantages of concave wheels 

 overbalance their advantages. Mr. Anftice alfo,in his"Trea- edge of the wheel B F, and then the load overfets the wag- 

 tife on Wheel-Carriages," whilft he recommends concave gon. Mr. Beighton has off'ered feveral reafons to prove, that 

 wheels, candidly allows, that fome difadvantages attend this the axles of wheels ought not to be ftraight : for which we 

 conftruftion of them ; for the carriage thus takes up more muft refer to Defaguliers' Exp. Phil. vol. ii. Appendix, 

 room on the road, fo that it is more unmanageable ; and p. 540, &c. Moreover, if the axle were not at right angles 

 when it moves upon plane ground the fpokes not only do to the pole or ftiaft, but this was on one fide, then the 

 not bear perpendicularly, by which means their ftrength is coach or carriage would be drawn on one fide, and almoft 

 - - . -.-. . . ' ■ -^- all the weight would bear upon one horfe. With fome 



mechanics, it is a practice to bend the ends of the axle-trees 

 forwards, and thus make the wheels wider behind than be- 

 fore. Mr. H. Beighton maintains, that wheels in this pofi- 

 tion are more favourable for turning ; fince, when the wheels 



leffened, but the friftion upon the nave and axle is made 

 unequal, and fo much the more as they are the more diftied. 

 Dr. Brewfter farther (hews, that they are more expenfive, 

 more injurious to the roads, more liable to be broken by ac- 

 cidents, and lefs durable in general, than thofe wheels in 



which the fpokes are perpendicular to the naves. From are parallel, the outermoft would prefs againft the Imch-pin, 

 thefe and other confiderations, our author is decidedly of and the innermoft would rub againft the (lioulder of the 

 opinion, that if wheels are to be compofed of naves, fpokes, axle-tree. In reftilineal motions, however, thefe converging 



and felhes, the rim fhould be cylindrical, and the fpokes 

 perpendicular to the naves ; whereas in concave wheels, the 

 rims are uniformly made conical, which fubjefts them to a 

 variety of difadvantages. Every cone that is put in motion 

 upon a plane furface will revolve round its vertex, and if 

 force is employed to confine it to a ftraight line, the fmaller 

 parts of the cone will be dragged along the ground, and 

 the friftion greatly increafed. Now when a cart moves 

 upon conical wheels, one part of the cone rolls while the 

 other is dragged along, and though confined to a reftilineal 



wheels occafion a great deal of friction, both on the axle 

 and the ground, and muft therefore be more difadvantageous 

 than parallel ones. This faft is allowed by Mr. Beighton : 

 but he feenis to found his opinion upon this principle ; that 

 as the roads are feldom ftraight lines, the wheels fhould be 

 more adapted to a curvilineal than to a reftilineal motion. . 

 5. Large wheels are always more advantageous for rolling 

 than fmall ones, in any cafe, or upon any ground whatever. 

 If we confider wheels with regard to their frittion upon the 

 axles, it is evident, that fmall wheels muft turn as much of- 



direftion by external force, their natural tendency to revolve tener round than the large ones, as their circumferences are 



round their vertex occafions a great aod continued friftion lefs ; and, therefore, a wheel which is twice as large as an- 



upon the linch-pin, the ftioulder of the axle-tree, and the other will have twice the advantage in refpeCt of the fric- 



fides of deep ruts. tion, the holes of the naves and axles, and the weights upon 



Dr. Brewfter has made fome farther obfervations on the them, being equal. Again, if we confider the wheels as 



conftruftion of certain parts of the wheels. The iron plates, they fink into the earth, or fall into holes, the bearing of 



he fays, of which the riras are compofed, ftiould never be the great wheel being double that of the fmall one, it would 



lefs than three inches in breadth, as narrower rims fink deep fink but half fo deep ; and if the fmall wheel fhould meet 



into the ground, and therefore injure the roads and fatigue with a hole of the fame diameter with itfclf, it would wholly 



the horfes. See the fequel of this article. fink in, whilft only a fegment lefs than half of the great 



1 2 wheel 



