August 4, 1910] 



NATURE 



161 



{ same in every case, the machine enables an estimate to be 

 obtained of the relative power of road stones to withstand 

 the wear and tear of traffic, and the rubbing action which 

 takes place at the surface of a macadamised road. A series 

 of tests with this machine was made a few years ago in 

 connection with an examination of the constitution of the 

 stones by H.M. Geological Survey, and the results were 

 published in a work entitled " Attrition Tests of Road- 

 making Stones." The table given below shows the average 

 loss per cent, of a few typical rocks used for road- 

 making : — 



Percentage 



of loss in 



Stone (Quarry or Locality dust 



Quarlzite Wick, Glos 4'o 



Ferrtiginous Quartzite ... Winford, Somerset 4'4 



Quartz Porphyrite (luenast, Belgium 37 



Quarlzite Cneibourg 57 



Biotilehornblende Granile Mount Sorrel, Leicester- 

 shire 6'6 



Chalk Pit Flints t", rays, Essex 104 



Gabbros St. Sampson's Guernsey 107 



Calcareous Sandstone Liphook, Hants I7'3 



Foraminiferal or Mendip 



Limestone Winford, Somerset ... 19 



It will be seen from this table that quartzites stood the 

 test best, that flints came out fairly well, and that sand- 

 stones and limestones are at the bottom of the list. Though 

 the final test of a road metal can only be known by wear 

 and tear upon the actual road, yet the results obtained by 

 the systematic testing of stones under precisely the same 

 conditions serve as a guide in the selection of suitable 

 materials. They give no indication, however, of resistance 

 to crushing. Flints only lose a small percentage of their 

 weight by rubbing together, but they are crushed into dust 

 by heavy traction-engines. 



There can be little doubt that poor materials, with road 

 sidings used for binding, are largely responsible for the 

 unsatisfactory condition of many roads. .'\ limestone road- 

 metal is undesirable for most districts, and flints make bad 

 roads when they are used where heavy traction-engine 

 traffic occurs. In the long run it is less expensive to use 

 a good road-metal than a cheap one. It does not seem to 

 be recognised that good material can be carted as cheaply 

 t as bad, and that, properly applied, the former lasts years 

 longer than the latter. 



Assuming that a macadamised road has been properly 

 constructed, it is worth while to consider the chief causes 

 of damage to it. It is often said that motor-cars are 

 responsible for the chief part of the damage ; but that is 

 not really the case. If a newly made road be noticed, it 

 will often be seen that the ordinary motor-car traffic 

 scarcely wears the road at all in the tracks where the 

 wheels go, whereas the part where the horse traffic goes 

 is worn hollow in the middle, being dug out by the hoofs. 

 Motor-cars probably do less damage to a good road than 

 horses. Moreover, the damage done by a 2-ton pneumatic- 

 tyred pleasure car is superficial compared with that done 

 by a motor-waggon with a total weight of 12 tons. The 

 wear and tear is caused by (i) the heavy weight per axle 

 carried ; (2) the speed at which the heavy motor-car runs. 

 Under the Heavy Motor-car Order, a car weighing 3 tons 

 unladen, and having a load of more than 5 tons (making 

 a total of above 8 tons), must not exceed a speed of five 

 miles an hour, with or without trailer ; but this weight 

 and speed are constantly exceeded. It is the combination 

 of illegal speed with illegal weights carried that is largely 

 responsible for much serious wear and tear of roads. 



It is generally believed that ordinary motor-cars cause 

 much damage to roads by what is described as the " suck- 

 ing action " of pneumatic tyres ; but this action has never 

 been proved to exist. The action which undoubtedly does 

 remove the small particles of the road is due to a scouring 

 or brushing of the surface by the tyres, thus leaving the 

 large particles to be crushed into dust by the rigid wheels 

 of other vehicles. In the case of steel-studded pneumatic 

 tyres the brushing action is, of course, greatly increased, 

 and is accompanied by crushing forces. To sum up, the 

 causes of damage due to altered conditions of traffic are : — 

 (il Traction engines: great weight of engine; excessive 



NO. 2127. VOL. 84] 



vibration, rigid and ribbed construction of tyres. (2) Heavy 

 motors with trailers : vibration, weight, rigid tyres running 

 over road at high speed. (3) Pleasure cars : scouring action 

 of the pneumatic tyres of cars travelling at high speed. 



The chief cause of dust, as apart from its method of 

 production, is to be found in the use of unsuitable road 

 material. As already stated, the horse is a serious factor 

 in the creation of dust; and the 5-cwt. battering-ram, as 

 each leg of the horse has been called, gives a road a 

 succession of heavy blows, apart from the screwing or 

 puddling action, and disintegrates the surface far more than 

 is generally realised. The motor-car, on account of its 

 tyres and the rapidity of its movement, though it raises 

 and scatters dust to a greater extent than any other vehicle 

 when going very fast, does far less to create it than is 

 generally imagined. 



The dust nuisance may be lessened greatly by using 

 nothing but high-class road metal, reducing the quantity 

 of binding material, and reducing the cross-fall or camber 

 of the road, so as to ensure that the traflic spreads itself 

 over the whole width instead of always being driven to the 

 crown of the road ; but Mr. Walker Smith, in an ex- 

 haustive book recently published on " Dustless Roads and 

 Tar Macadam," points out that even when these conditions 

 are satisfied no very substantial improvement can be looked 

 for. " Even when the best material and the best methods 

 of binding are introduced, the road will always remain a 

 pervious road. The moisture in wet weather, which tends 

 to hold the particles of the road together, will, on being 

 evaporated in the dry weather, leave the surface loose and 

 friable and a ready prey to the disintegrating forces, the 

 shock of the horses' feet, the abrasion of the steel-studded 

 vehicle, and the scouring action of the soft-tyred ones." 

 Mr. Walker Smith says very strongly that the binding is 

 undoubtedly the crux of the whole question of efficient 

 road-making and maintenance. The binding makes or 

 mars the macadamised road, and it is, and ever has been, 

 the weak spot in the ordinary macadam road. The Dust 

 and Dustless Roads Committee of the Royal Automobile 

 Club reports that, in the opinion of the whole of the road 

 engineers Vith which it has been in touch, if macadam 

 roads are to be constructed to meet the needs of the present- 

 day traffic, with the searching demands that the traffic 

 makes on the road surface, a bituminous binding or matrix 

 must be employed. 



The committee states that, setting aside the temporary 

 palliatives of watering roads with chemical preparations 

 which keep the road damp by the absorption of moisture 

 from the air, the treatment which has been most successful 

 in rendering roads dustless has been the surface apphca- 

 tion of tar applied either by hand or by machine. By this 

 method great lengths of road have been rendered dustless 

 for a whole summer season, the mud in the winter follow- 

 ing has been reduced in quantity, and in some cases the 

 application has lasted for more than one season. More- 

 over, there is almost unanimous testimony that the whole 

 cost of the treatment is more than saved by the increased 

 durability of the road, and already many surveyors are able 

 to make' a strong case for the extension of the treatment 

 solely on the ground of economy alone. 



The most permanent mode of treatment is that of re- 

 making the whole of the surface of the road with tar 

 macadam, and when a road has to be re-surfaced this 

 treatment is also the most economical in the end. 



The use of calcium chloride to keep down dust is not 

 recommended, .one reason being that the keeping of the 

 roads moist and soft tends to the more rapid wear of the 

 surface. 



The Roads Improvement Association has issued a report 

 shoAving the extent and result of the treatment of roads 

 bv tanking. From this it appears that the road must be 

 thoroughly cleansed before treatment ; about six square 

 yards of surface can be treated per gallon of tar ; sand or 

 granite chippings must be applied after treatment ; the 

 average cost is 'about li. or i\d. per square yard when a 

 machine is used, and \\i. to \\i. when the tar is applied 

 by hand ; at least one treatment per annum is required ; 

 the road should be dry when treated. Excellent results 

 have been obtained at the cost of about 40/. per mile, and 

 with a small consumption of tar — about one-sixth to one- 

 fifth of a gallon per square yard. 



