460 OUR COMMON ROADS—MURPHY. 
width of the tires when it exceeds 3 to 4 inches, but on compres- 
sible surfaces it decreases in proportion to the width of the tire. 
The resistance increases with velocity on hard roads, but is 
independent of velocity on soft surfaces. 
Springs diminish the resistance at high speeds, but not at slow 
speeds. 
From trials made with a dynomometer attached to a wagon, 
moving at a slow pace upon a level, the following table gives the 
force of traction in pounds upon several kinds of road surfaces 
in a fair condition, the weight of the wagon and load being one 
ton of 2.240 pounds : 
1.) On best stone trackways (7.0.0.0 124 povnds 
2. A good plank road..-........2.- 32 to 50 r 
3. <A cubical block pavement........ 32 to 53 " 
4. A macadarized road, small }roken 
SEO ners hoe cere oust otod cent e Mee ene 65 " 
5. A Telford road, made with broken 
stone on a pavement......... 46 " 
6. A road covered with 6 inches of 
broken stone cr concrete foun- 
GAtIOM: 0255 ether oars eee 46 " 
7. A road made with a thick coating 
of gravel laid on earth... ...140 to 147 " 
S. SA’eommon earth road. ss... os. . 200 " 
In order to apply these results in establishing suitable grades 
take the case of the macadamized road, No. 4, in which the 
tractive force is 65 pounds on a level road: 
Let W-==the weight of the vehicle and load in pounds. 
p=pressure normal to the road surface. 
t—force of traction in pounds on a level road. 
At the angle of repose of an inclined road, the force, acting 
parallel to the line of grade, necessary to sustain a carriage and 
its load in its position on the incline, or to prevent its moving 
back by its own weight, is equal to the traction force t, which 
would just move the carriage and load on a level road. Leth be 
