92 



NA TURE 



[July 22, 1909 



feeders, and the chapter is finished with a few words 

 on weighing and sampling. 



There is a great deal of varied information con- 

 tained in the bool<, but it would have been better to 

 confine the subject to one or other of the branches 

 referred to in the opening paragraph. 



DUST LESS ROADS. 



Dustless Roads. Tar Macadam : A Practical Treatise 

 for Engineers, .Surveyors, and Others. By T. 

 Walker Smith. Pp. xi + 225. (London: Charles 

 Griffin and Co., Ltd., 1909.) Price los. 6d. net. 



THE growing use of motor-cars, and the destruc- 

 tive action due to the rapid speed at which they 

 are driven, and the sucking action of the indiarubber 

 tyres on the surface of the roads, has rendered a 

 change in their management necessary. 



It is the almost unanimous opinion of road experts 

 that, in order to preserve the surface of the roads in 

 good condition, it is necessary that some kind of 

 bituminous material must be used for binding the 

 stones of the macadamising, which, while cementing 

 them together, will also give a waterproof and com- 

 paratively dustless surface. 



The book under notice has. therefore, been pub- 

 lished at an opportune time, as the method of repair- 

 ing roads described in it, which has been more or less 

 successful, has been so far only of a tentative 

 character, the work not being carried out on any 

 scientific basis, but very much by the rule of thumb. 



The author, who was borough engineer at Barrow- 

 in-Furness for several years, had under his charge 

 many miles of macadamised roads subject to motor 

 traffic, and devoted his attention to experimenting and 

 trying to find out the best means of solving the difii- 

 cult problem of keeping these roads in good order, and 

 preventing the creation and spreading of dust, at a 

 reasonable outlay. The results of his experience are 

 given in the book under notice. The author, how- 

 ever, not content with his own experience, has also 

 collected the opinions of a large number of road 

 surveyors throughout the country as to the advan- 

 tages and disadvantages, and cost of tar macadam, 

 the answers to the queries submitted being given in 

 the tables contained in the book. 



The subjects dealt with are divided into fifteen 

 chapters 1 elating to tar macadam as a remedy for dust 

 nuisance; the necessity for standardisation in con- 

 struction ; tar ; aggregates for tar macadam ; prepara- 

 tion and laying; mechanical mixing; effect of wear 

 and tear; scavenging, watering, and maintenance; 

 camber, gradient, noiselessness, and hygienic advan- 

 tages ; tractive effort ; statistics of road mileage ; cost 

 of maintenance ; and tar spraying. There are twenty- 

 four illustrations and a tabulated analysis of the 

 replies to queries. 



The author shows that the binding material used 

 in the making or repair of macadamised roads is the 

 crux of efficient road maintenance and the prevention 

 of dust. His opinion is that it is absolutely necessary, 

 if macadamised roads are to meet the needs of present- 

 NO. 2073, VOL. Si] 



day traffic with the searching demands that motor 

 traffic makes on the surface of the road, that a 

 bituminous binding or matrix should be employed. 

 He points out that so far, although this fact is fully 

 recognised, no scientific study has been made to 

 standardise the materials used, and the proportion and 

 quality of this material : — 



" Thus the matrix is usually prepared as it seems 

 best to the individual in charge of the tar boiler who 

 uses anything that comes from the local gas works ; 

 who boils it as long as he thinks necessary, and adds 

 to it whatever he thinks may improve it in the way of 

 pitch or creosote. It is a subject in which the per- 

 sonal factor has entered to an exceedingly great 

 extent, and each man acts more or less as it seems 

 best to him in his own mind." 



The aggregates for tar macadam are also fully 

 dealt with. The author points out that as the tar 

 binding ceme.its the stones together, the abrasion 

 caused by friction in water-bound macadam is thus 

 absent, the properties of noiselessness, elasticity, and 

 resilience being secured. The only portion of the road 

 material which is subject to any considerable wear is 

 \he surface, which has to bear the friction due to the 

 rolling of the wheels, and also the impact of the 

 horses' feet. It is, therefore, contended that the use 

 of granite or other similar expensive material is not 

 necessary for the lower coat, which consists of from 

 80 to 85 per cent, of the whole; but that where the 

 road is only subject to moderate traffic the lower layer 

 may consist of hard limestone or slag, either of which 

 material holds the tar better than granite ; the upper 

 layer, which has to bear the surface wear and tear, 

 being composed of granite. The author's experience 

 leads him to the conclusion — 



" that it is an absolute waste of money to put such 

 good material as granite in the bottom when a less 

 costly material will do perfectly well when armoured 

 with a good coating of tarred granite." 



The patent processes known as " Tarmac " is de- 

 scribed. The makers of this road material have 

 expended upwards of 2o,oooJ. in the construction of 

 works and plants at Wolverhampton. The aggregate 

 used is slag, the tar is distilled before using, and the 

 mixing is done by machinery. The author, however, 

 does not seem to think that it has any special advan- 

 tage over ordinary tar macadam mixed locally when 

 this is done in a proper manner. In the tables the 

 cost of the materials and of mixing and laying is 

 given for a great number of localities. As an average 

 this may be taken as from two shillings to half a 

 crown per square yard for a coating of 35 inches of 

 slag and hard limestone for the bottom layer, and half 

 an inch of tarred granite for the surface coat. The 

 general opinion appears to be that there is a saving in 

 the cost of maintenance of roads where tar macadam 

 is used in place of water-bound macadam, in some 

 cases amounting to as much as 75 per cent., the 

 average, however, being put at 37J per cent. 



With occasional tarring and sanding of the surface, 

 a tar-macadam road, so far as the lower layer is 

 concerned, is practically everlasting. The surface 



