560 ON DRAUGHT. 



The fact of coaches thus loaded miming light, has been clearly proved by tho 

 failure of what were called Safety Coaches, in which the only difference con- 

 sisted in placing the load very low. These coaches, although completely 

 answering their purpose of safety, were discontinued solely, we believe, from 

 their being found destructive of the horses. 



Experiments, nevertheless, have been made to prove that this was only an 

 idle prejudice of coachmen ; but universally received opinions, even if leading 

 to erroneous conclusions, generally have some good foundation ; and coachmen, 

 although they may not have been so much so at the time these experiments 

 were published (in 1817), are certainly now rather an intelligent class of men. 

 We should, therefore, prefer risking a theory, if a theory were necessary, in 

 support of their prejudices rather than in opposition to them. The experiments 

 alluded to were not, in our opinion, made under the circumstances which occur 

 in practice. Small models (the wheels being seven inches in diameter) were 

 drawn along a table across which were placed small strips of wood to represent 

 the obstructions met with in a road ; but these strips of wood came in contact 

 with each pair of wheels at the same time, and never caused any lateral motion. 

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

 in a road, where the action rarely affects more than one wheel at a time ; con- 

 sequently, in the model, the wheels, in passing over an obstruction, 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 opinions 

 of those whose daily experience ought to enable them to judge correctly. 



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, 

 in the latter case, produce quite a contrary effect. 



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

 from 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, is only true in 

 the case of low velocities and carriages without springs. 



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

 conveying the weight, and shown in what manner they affect the draught, and 

 how they should be constructed so as to diminish as much as possible the amount 

 of this draught. We have endeavoured to point out the advantages and neces- 

 sity of attending to the construction and size of the wheel. Thus 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 firm, unyielding con- 

 nexion 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 in movement, should always 

 act in the same manner, without elasticity, in propelling the wheels ; and 

 lastly, that it is highly advantageous 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 rise and fall over stones or irregularities in the road without 

 communicating any sudden shocks to the load ; and we believe that the proper 

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

 productive of great good effect. 



