538 POPULAR SCIENCE MONTHLY, 



condition iniless every defect whicli may develop is immediately re- 

 paired. When the smooth, hard surface is once broken, disintegra- 

 tion proceeds very rapidly, and a large hole is soon formed. The 

 more general distribution of smooth pavements, however, will tend 

 to distribute the traffic more evenly, and the increasing use of 

 rubber tires and rubber shoes for horses, to say nothing of the prob- 

 ably quite general use of motor vehicles, within the next decade 

 will result in the elimination of the forces at present most destruc- 

 tive to pavements. 



Much regret is often expressed that asphalt pavements should 

 be so frequently opened for the purpose of laying or obtaining 

 access to subsurface pipes and conduits, and thereby mutilated. 

 As a matter of fact, there is no pavement at present in use 

 Avliich can be so effectively and satisfacortily restored as asphalt. 

 AVhen skillfully done, almost no trace of such an opening can 

 be found. 



The first question to arise, when it has been determined to pave 

 a street, will be the selection of material, or the kind of pavement 

 to be laid. In determining this, the governing considerations will 

 be the traffic to be sustained, its density and character, the rate of 

 grade, and the presence or absence of railroad tracks. 



If the traffic be very heavy and the street given up wholly to 

 business, ease of traction, durability, and economy of maintenance 

 are of first importance, while quiet, comfortable riding, and beauty 

 can be sacrificed to them. Many efforts have been made to deter- 

 mine the relative force required to draw a load over different kinds 

 of surface under similar conditions. The following is from a table 

 compiled by Mr. Rudolph Hering, from different authorities, the 

 force being that necessary to move one ton on -a level grade at a 

 speed of three miles an hour: 



Kind of Road. Pounds'. 



Ordinary dirt road 224 



Ordinary cobblestone 140 



Good cobblestone 75 



Common macadam fi4 



Very hard, smooth inacudaii] 46 



Good stone block 45 



Best stone block (London) 36 



Asphalt 17 



(Jraiiitc tiani way 1 2^ to 1 3^ 



Iron railway 8 to 1 H 



The question of durability occurs next, and the different kinds 

 of pavement which may be considered for city streets may be 

 rated as follows, it being assumed that the traffic is not excessively 

 heavy: 



