PAVEMENTS AND TRACKWAYS. 241 



in the "winter. There seems to be a sort of notion that wood 

 pavements and coal tar must go hand in hand ; but there cer- 

 tainly is no necessity for this. Coal tar is applied as a preserv- 

 ative to the wood ; but it must be acknowledged that many 

 better ones are known and indeed are used, to the utter exclu- 

 sion of coal tar, in all cases where it is desired to preserve wood, 

 except in this of wood pavements. No wood should be used in 

 paving that has not been first subjected to some approved method 

 of preservation, or impregnation, as it is frequently called. The 

 best manner of setting the same is still a mooted point, which 

 it would be presumptuous at present to decide. 



Cast-iron pavements are out of favor on account of their 

 great cost, and concrete pavements are a matter of experiment 

 as yet. 



Asphalt pavements are chiefly used in Paris. They are slip- 

 pery in wet weather, and produce a very disagreeable, pene- 

 trating dust in dry weather. It is necessary to prepare a bed of 

 macadam to lay them on, and they are not used in Paris except 

 in streets where the gas- pipes are carried either in the sewers or 

 under the sidewalks, as any leak of gas would destroy them. 

 Their use is a matter of doubtful economy. 



Trackways are, as has been mentioned, out of date. Where 

 a common road does not suffice now-a-days, a railroad is built ; 

 but time was when trackways were of considerable importance. 

 They consist, if of stone, of large, flat stones, say 12" deep and 

 4-6 feet long by 14"-16" wide, solidly bedded in two parallel 

 rows, at such distance apart as to make of each row a track for 

 the wheels. The space between is paved. They are of course 

 very expensive, but cost little to repair, and enable a horse to 

 pull a very great load. As has been mentioned, Telford made 

 use of such a stone trackway, to avoid cutting down a hill, on 

 his Holyhead road. There were two hills, each a mile in 

 length, with an inclination of 5 in a hundred. It would have 

 cost ilOO,000 to reduce this grade to 4^ in a hundred, but 

 nearly the same advantage, in diminishing the tractive force re- 

 quired, was obtained by keeping the 5 in a hundred grade, with 

 moderate cuttings and embankments, and making stone track- 

 ways, at a total expense of less than half the former amount. 



" Plank roads," once so much in vogue in the United States, 

 may not improperly be classed among roads with trackways, 

 31* 



