The Construction and Maintenance of Roads. 



By ALFEED J. BURROWS, Pluckley, Kent. 

 {Continued from page, G20.) 



The cnlire subject of road-making and maintenance is one of such 

 great importance that some remarks upon the theory may not be out 

 of phice, especially as recent experiments and practice have done so 

 much to dispel old ideas, and establish new ones based on science. 



As distances are measured by units of length, — the inch, foot, or 

 yard,- — capacity by gallons, bushels, or chaldrons, — weight by ounces, 

 pounds, or tons, — so the unit of work of the engineer is the force 

 required to lift one pound in weight through one foot of space. By 

 elaborate calculations Watt fixed the power of a horse at 38,000 units 

 of work per minute, though later calculations place it much lower, and 

 consider 22,000 a fair average. Still the former figures are adopted 

 by engineers in fixing the horse-power of their engines. 



The force necessary to move a load is due partly to the resistance of 

 friction and partly to gravity. Friction diminishes in proportion to 

 the smoothness and rigidity of a road : thus the force necessary 

 to move one ton upon the metals of a railway is about Tr|^th part 

 of its weight, or eight pounds ; whereas to move the same load upon 

 the best macadamized road requires a pressure of about 7jVth, or 74 

 pounds. The force required in each case is called the traction, and 

 the fraction by which it is expressed, its co-efficient of friction. 



In calculating the power of a horse to move a load upon a rising 

 road, to the resistance of friction must be tidded the work necessary 

 to overcome gravity. Supposing the rise of the road to be 1 in 20 ; 

 for every 20 feet travelled by the horse he will lift the entire load to 

 the height of one foot, or have performed 2,210 extra units of work — 

 equal to 112 units for every foot run. To perform his quota of work 

 according to the theory of Watt, the horse must draw his load upon 

 the level at the rate of 440 feet per minute, or proceed at the rate of a 

 mile in 11 minutes; while upon the road having an incline of 1 in 20 

 he would require to proceed only at the rate of 117 feet per minute, or 

 a mile in about 30 minutes. 



As a somewhat exaggerated illustration of the unnecessary exj)en- 

 diturc of power in dragging loads up steep liills, let the i'gader take a 



