. Les autoroutes américaines, un compte rendu populaire de leurs conditions et des moyens par lesquels elles pourraient être trahis, on n'a pas été mis en place dans de telles conditions afin d'assurer un lien approprié pour unir les fragments dont elles sont composées. Dans de nombreux cas, les dommages sont dueto le fait que le soin n'a pas été pris pour assurer une propriété où la route est posée sur l'argile. Comme beforenoté, où l'argile sous une route n'est pas parfaitement gardée sèche la structure est toujours en péril. Il y a un autre genre de violence que les roues font à aroad qui est provoqué par le besoin de freinage ou, ce qui est pire, de les bloquer comme

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. American highways; a popular account of their conditions, and of the means by which they may be bettered . one was not put on under such conditions aswould insure a proper bond to unite the fragments ofwhich it is composed. In many cases the damage is dueto the fact that care has not been taken to secure a properfoundation where the road is laid upon clay. As beforenoted, where clay beneath a road is not kept perfectly drythe structure is always in peril. There is another kind of violence which wheels do to aroad that is brought about by the need of braking or, what is worse, locking them as a laden vehicle goes down 166 AMEBIC AN HIGHWAYS a steep grade. Either of these means of hindering thedescent of the vehicle inflicts damage in proportion to theextent to which the free turning of the wheel is resistedand its movement converted into sliding. If the bond ofthe stones is exceptionally firm the action may be limitedto a very rapid surface wearing, probably some score timesas great as where the wheel is free. If, however, the unionof the stones is weak, as it always is either in wet weather. Fig. 13. —Showing wagon-wheel sustained at sin-gle point of contact on hard, smooth surface ofcompact Macadam or Telford road. or in very long droughts, the effect is to tear out the bits.As in the matter of narrow tires, it seems useless to seekto remedy this evil by laws, and this particularly for thereason that where the load is held back by the pressure ofthe horses on the breeching the calks of their shoes arelikely to do about as much damage as the checked or lockedwheels. If the grades are too steep the road must takedamage from the defect of the plan; it matters little inwhich way it is inflicted. The remedy is to diminish the METHODS OF CONSTRUCTINa ROADS 167 slope to the point where the road will not tend to acquire inits downward conrse a velocit}^ which needs to be checkedby any considerable exercise of power; for all the energythns nsed is directly applied to the

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