ON DRAL .xiT. 447 



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

 peripheries of the wheels should, in their transverse section, be as nearl* 

 as possible tangents to this curve ; but since no exact form can be assigne<l 

 (o 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 grad- 

 ually 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 the 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 pov/er is diminished at any 

 inclination 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 inclination upward, for the purpose of acting with 

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

 different animals exert their strength most advantageously in ditferent direc- 

 tions; and, therefore, practice can alone determine what precise inclination 

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

 tions are, however, only applicable to cattle drawing immediately at the 

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

 tion 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 concussions 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 weigbt 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 applica.- 

 rion 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. All 

 stage-coaches, and many travelling carriages, hang upon grasshopper 

 springs, which allow of perpendicular without any longitudinal action. 

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

 pending post-chaises were a little more a.ttended to. The more elasticity, 

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



