OM DRAUGHT. 447 



* Roads must have some degree of curvature to throw off water, and the 

 peripheries of wheels should, in their transverse section, be as nearly as 

 possible tans^ents to this curve ; but since no exact form can be assigned to 

 roads, and they are found to differ almost from mile to mile, it is pre- 

 sumed, that a small transverse convexity given to the peripheries of 

 wheels, otherwise cylindrical, will sufficiently adapt them to all roads ; and 

 that the pressure of such wheels, greatest in the middle, and gradually 

 diminishing towards the sides, will be less likely to disarrange ordinary 

 materials, than a pressure suddenly discontinued at the edges of wheels 

 perfectly flat. 



' The spokes of a wheel should be so arranged, as to present themselves 

 in a straight line against the greatest force they are in common cases likely 

 to sustain. These must evidently be exerted in a direction pointed towards 

 the carriage, from lateral percussions, and from the descent of either wheel 

 below the level of the other : consequently, a certain degree of what is 

 termed dishing, must be advantageous, by adding strength ; whilst this 

 form is esteemed useful for protecting the nave, and for obviating the ill 

 effects of expansions and contractions. 



' The line of traction is theoretically best disposed, when it lies exactly 

 parallel to the direction of motion ; and its power is diminished at any in- 

 clination of that line, in the proportion of the radius of the wheel to the 

 cosine at the angle. When obstacles frequently occur, it had better, per- 

 haps, receive a small inchnation upward, for the purpose of acting with 

 most advantage when these are to be overcome. But it is probable, that 

 dilFerent animals exert their strength most advantageously in different di- 

 rections ; and, therefore, practice alone can determine what precise incli- 

 nation of the line is best adapted to horses, and what to oxen. These con- 

 siderations, are however, only applicable to cattle-drawing immediately at 

 the carriage ; and the convenience of their draft, as connected with the 

 insertion of the line of traction, which continued, ought to pass through 

 the axis, introduces another limit to the size of the wheels. 



* Springs were in all likelihood first applied to carriages, with no other 

 view than for the accommodation of travellers : they have since been found 

 to answer several important ends. They convert all percussions into mere 

 increase of pressure ; thus preserving both the carriage and the materials 

 of the roads from the effect of blows ; and small obstacles are surmounted 

 when springs allow the frame and wheels freely to ascend, without sensibly 

 moving the body of the carriage from its place. 



' If the whole weight is supposed to be concentrated on springs very 

 long, extremely flexible, and with the frame and wheels wholly devoid of 

 inertia, this paradoxical conclusion will most certainly follow : that such a 

 carriage may be drawn over the roughest road without any agitation, and 

 by the smallest increase of force. 



' It seems probable that springs, under some modification of form and 

 material, may be applicable with advantage to the heaviest waggon.' 



And there can be no doubt, that, in the words of the writer, the appli- 

 cation of springs would be highly advantageous. At high velocities, as 

 we have before said, .the effect of springs is still greater. What we have 

 instanced as regards springs, is generally Well known and understood. 

 AH stage-coaches, and many travelling carriages, hang upon grass- 

 hopper springs, which allow of perpendicular without any longitudinal 

 action. It would be much to the interests of horse masters if the mode of 

 suspending post-chaises were a little more attended to. The more elas- 

 ticity, or in other words, the more action, there is in grasshopper springs, 



