•'"'^'^ ON DKAUGHT. 



made under the cii^cumstances which occur in practice. Small models rthe 

 wheels being seven inches in diameter) were drawn along a table across 

 which were p aced small strips of wood to represent the obstructions met 

 mth m a road ; but these strips of wood came in contact with each pair 

 ot wheels at the same time, and never caused any lateral motion They 

 produced, therefore, a totally different effect from that which takes place 

 m a road, where the action rarely affects more than one wheel at a time • 

 consequently, m the model, the wheels, in passing over an obstniction,' 

 threw the whole weight backwards in a direction exactly opposite to the 

 movement required; while, in practice, the carriage is generally thrown 

 sideways, which does not affect its forward motion. 



The conclusions drawn from these experiments are, therefore, as might 

 be expected, at variance with practical results, and directly contrary to the 

 opmions of those whose daily experience ought to enable them to iud^e 

 correctly. j & 



_ The effects, also, of velocity and momentum must be difficult to imitate 

 in models. 



The advantage of placing the load high will not, however, equally apply 

 at low velocities, still less when springs are not used ; it may frequently 

 indeed, m the latter case, produce quite a contrary effect. ' 



In a rough road, the increased force with which the load would be thrown 

 trom side to side might prove very inconvenient, and even dangerous, and 

 would certainly be liable to increase the resistance when the front wheels 

 meet with any obstruction ; but this, it must be particularly remembered 

 ^^ w -^1, ^^ ^^^^ °^ ^"^ velocities and carriages without sprino-s 



We have now considered in succession the various parts of the vehicle 

 tor conveying the weight,and shown in what manner theyaffect thedrauo-ht 

 and how they should be constmcted so as to diminish as much as possible 

 the amount of this draught. We have endeavoured to point out the advan- 

 tages and necessity of attending to the construction and size of the wheel 

 ihus it should be as nearly cylindrical and vertical as possible, and of as 

 large a diameter as can conveniently be admitted. 2dly, That there should 

 be a hrm, unyielding connection in the direction of the movement between 

 the power employed, the weight moved, and the wheels : in other words 

 that the force should always act directly and without elasticity both upon 

 the load and upon the wheels ; and that the impetus or momentum of the 

 load when m movement, should always act in the same manner, without 

 elasticity, m propelling the wheels ; and lastly, that it is highly advan- 

 tageous to interpose as much elasticity as possible by means of springs in 

 a vertical direction between the wheels and the body, so that the former 

 may nse and fall over stones or irregularities in the road without commu- 

 nicating any sudden shocks to the load ; and we beHeve that the proper 

 application of springs in all cases, even with the heaviest loads, would be 

 found productive of great good effect. 



Attention to these points will tend to diminish considerably the amount 

 of draught. As far as regards friction at the axles, and the resistance in 

 passing over obstacles in the road, it will assist the favourable appKcation 

 ot the force of traction when obtained from animal power ; but that which 

 we have shown to be the most considerable source of resistance is unfor- 

 tunately least affected by any of those arrangements. We allude to the 

 resistance arising from the yielding or crushing of the material of the road • 

 we have seen that on a good turnpike-road the draught was increased iA 

 the proportion of thirty to forty, or about one-third, by the road being 

 sligMfy dirty ; and that, on a heay^^, sandy road, the di^aught was increased 

 to Mi>, or nearly seven times. Springs will not affect this ; and even in- 

 creasing the diameter of the wheel will be of very slight assistance ; nothin«^ 



