^56 OX DRAUGHT. 



scarce, and probably at best merely tracks, much could not be expected 

 Irom vehicles constructed under such circumstances ; the wheels ^yere 

 small, from twenty to thirty inches diameter, and all the parts of the cha 

 riots excessively heavy, so as to resist the repeated shocks to which thev 

 were subject, -^ 



The chariots represented upon the Frieze of the Parthenon, before 

 alluded to, and which is probably upwards of 2200 years old, are verv 

 light m their construction, and only want springs to be called o-io-g 



iheadvancement of all the branches of the mechanical artslias neces- 

 sarily introduced many improvements in the detaUs of the construction of 

 the wheelitself, as well as in that of the axle and the rest of the carriao-e 

 and by this means no doubt increased very greatly the use and advantage 

 ot It ; but it IS a remarkable fact that these improvements have been con- 

 faned exclusively to the workmanship and mechanical detail, and that tho 

 principle has remained exactly the same, and has not even received any 

 addition during this immense lapse of time. 



Upwards of 3000 years ago, the wheels appear to have been independent 

 ot each other and running upon fixed axles ; we can say no more of the 

 most improved wheel of the most finished carriage of the present day 



VVe are far from intending to cast any slight on modern invention, or to 

 compare the groaning axle-trees and creaking wheels of the ancients with 

 the noiseless Cohnge's axles of the nineteenth century ; but truth compels 

 us to acknowledge that a period of thirty centuries, more than half the 

 time which is supposed to have elapsed since the creation of the world has 

 produced no radical change nor brought into action any new principle in 

 the use of the wheel as applied to carriages. 



The particular form and construction of the wheel, as well as of all the 

 other parts of the carriage, however, admit of great variety, and the 

 draught is materially aff'ected by their variation. We shall therefore 

 after examining the action of wheels in general, describe the mode of con- 

 struction now adopted, and then endeavour to point out the advanta^^es and 

 disadvantages of the various forms which have been given to the different 

 parts of it. 



First let us examine the theory of it, and suppose it acting on a level plane 

 ihe wheel being a circle, the centre will remain always at the same 

 height, and, consequently, will move parallel to the plane in a perfectly 

 level hne : if any weight be attached to or suspended from its centre this 

 wiU also move m a continued straight line without rising or fallino- and 

 consequently when once put in movement there is nothing to ch(^c'k its 

 progress (neglecting for the moment the shght resistance of the air), and 

 it will require no force to keep it in motion so long as the wheels continue 

 to turn. 



We have, therefore, in this case only to examine into the force necessarv 

 to turn the wheels. The wheels, if left to themselves, would roll on with 

 perfect freedom, whatever might be their weight, or whatever weight 

 might be attached to them, provided nothing in the mode of attaching that 

 weight impeded then- revolution ; but in practice we cannot admit of the 

 load revolving with the wheel, and we have no means of suspending it to 

 the wheel, except by means of an axle fixed to the load, and passino- 

 through the centre of the wheel. This axle presses upon the lower surface 

 of the hole, and consequently, when the wheel revolves, causes a friction 

 proportionate to the load upon the axle. This friction is then the only 

 source of resistance to the motion of a wheel, under the cu-cumstances here 

 supposed ; and it is the action of this friction, the degree in which this 

 affects the draught, and by what means this effect is increased and dimi- 

 lushed, that we are now about to consider. 



