DETERIORATION OF ROADS. 11 



insignificant. The moment, however, an irregularity of any kind 

 occurs on tin- surface (lie wheels begin to pound at such points and 

 water accumulates in the resulting depressions, which causes rapid 

 deterioration. When the pressure per running inch of tire is too 

 great for the road surface had results soon follow. With crushed 

 stone or gravel, not only should the pressure he within the limit of 

 resistance of the TOCk of which the road is composed, hut within the 

 cohesive limit of the binder surface, for if this is not the case the pa - 

 ing wheels cause the material immediately beneath them to shift, fur- 

 rowing the road and pressing the material up on either side of the fur- 

 row. This rutting is greatly increased by the consequent interference 

 with the proper draining to the sides of the road, and the water is 

 forced to accumulate on the surface, to gully out the road on grades, 

 or make its way to the foundation. Always on a good road the resist 

 ance offered to compression by the material composing it exceeds the 

 pressure of wheels, for if such were not the case the surface would 

 soon be cut to pieces. It is, however, a frequent occurrence for the 

 impact and even the pressure of wheels to exceed the limit of resistance 

 of the binder surface Qf macadam and gravel roads. The pressure of 

 wheels is distinctly beneficial to a road if it is sufficiently within the 

 cohesive limit of the binding material and is evenly distributed over 

 the surface. Indeed, it is necessary if a road is to be kept in a high 

 state of perfection that it should be subjected to a sufficient amount of 

 traffic to keep the material thoroughly compacted, and at the same time 

 wear off a sufficient amount of fine material to take the place of that 

 carried away by wind and rain. 



CHEMICAL AGENCIES. 



It has frequently been remarked by writers on the subject, that the 

 chemical decomposition of materials plays an important part in the 

 deterioration of roads. If wood and asphalt are used this may be the 

 case, but the writer is convinced that undue importance has been 

 attached to this agency as regards most of the road materials. In a 

 warm, dry climate there is practically no limit to the life of rock. 

 This fact is exemplified by many of the structures of antiquity, such 

 as the pyramids of Egypt, composed of limestone and granite, which 

 have stood for many centuries, while the rock has undergone but little 

 change. In such a climate this would be the case with almost any 

 variety of rock, but in a humid climate with a great range of tem- 

 perature less durability is to be expected. In all cases, however, the 

 durability of rock is extremely great as compared with the life of a 

 road. All rain and sin-face waters contain a sufficient amount of car- 

 bonic acid to dissolve carbonates of lime and iron, and when chained 

 with the humus acids derived from the decomposition of animal and 



