214 



DISCOVERY 



like a retired Clyde-steamer captain about to fill his 

 second pipe. 



The book is altogether a good one, and should be 

 welcomed by everyone who is privileged to take an 

 interest in science. 



In talking of the British Association's past we need 

 not forget it has a future too. In 1924 it hopes to 

 meet in Canada. This year it meets in HuU. It met 

 there once before, in 1853, with Hopkins the geologist 

 as president. This time Sir Charles Sherrington is in 

 the chair. His subject will be " Some Aspects of 

 Mechanism." Several of the addresses of the sectional 

 presidents will deal, as befits a meeting in Hull, with 

 North Sea problems. Among the subjects of joint 

 discussion will be " The Origin of Magnetism," " Psy- 

 cho-analysis and the School," " Mental Characters and 

 Race," " The Present Position of Darwinism," and 

 " The Possibility of Increasing the Food Supply of 

 Great Britain." Dr. F. W. Aston wiU give an evening 

 discourse on " The Atoms of Matter," and Prof. W. 

 Garstang one on " Fishing: Old and New." 



A. S. Russell. 



Modem Road-making in 

 Cities 



By George Whale 



The enormous increase in the weight of vehicular 

 traffic during the last ten years has awakened those 

 responsible for the maintenance and construction of 

 roads, both in the cities and the country, to the fact 

 that new methods must be adopted to meet the altered 

 conditions. Owing to the weight of the motor-omnibus, 

 the motor lorry and steam wagon now universal in 

 road transport, a good foundation and a solid one is 

 absolutely essential. In the cities, therefore, cement 

 concrete foundations are laid in all main thorough- 

 fares which are paved with wood blocks, granite setts, 

 or one of the various forms of asphalt, of which there 

 are several to choose from. 



Of these three types of paving the granite setts 

 are the most durable, but they are extremely noisy 

 and ill adapted for any streets except those in the 

 vicinity of docks or warehouses. Wood paving com- 

 pares very favourably with other types. It is easily 

 cleaned and is practically non-absorbent, and traffic 

 over it is more silent. It has a lengthy life and requires 

 little or no repairs for several years. As compared 

 with asphalt it is slightly inferior from a sanitary 

 point of view and a little less silent, but on the other 

 hand affords a much better foothold for horses and, 

 if it is properly cleansed, vehicles are less likely to 



skid on it. For these reasons wood paving finds most 

 favour with municipal engineers and is in general use 

 in the principal carriageways of London and other 

 large cities. 



The work of repaving a carriageway with wood is 

 one of considerable magnitude. The old blocks have 

 to be stripped and taken away and the old concrete 

 foundation broken up and removed. In its place a 

 new concrete foundation is laid, 12 in. deep for heavy 

 traffic and 9 in. deep for moderate traffic. Experiments 

 are also being conducted in introducing metal re- 

 inforcing, which reduces the thickness of concrete 

 required. The concrete foundations in the best class 

 of work are composed of clean sharp ballast or crushed 

 concrete and Portland cement mixed in a proportion 

 of 6 to I. 



Until quite recently all this work was carried out 

 by hand labour, but to-day several of the municipal 

 boroughs of London are employing concrete mixing 

 machines which, besides executing the work in a much 

 better manner, effect a considerable saving in labour. 

 These mixing machines move under their own power 

 and require a complement of fifteen men. They are 

 capable of spreading as much in one day as would 

 be carried out by sixty men mixing concrete by hand. 



The crushing machine, which is employed in crush- 

 ing the best of the old concrete that has been broken 

 out, is also a source of economy. On several road 

 works carried out recently about 50 per cent, of new 

 ballast was saved by using old crushed concrete, and 

 a similar saving in carting away of old material was 

 accordingly secured. On the top of the concrete 

 foundation is laid an average depth of i in. of fine 

 cement concrete, known as " floating," to afford a 

 smooth and level bed for the wood blocks. This 

 " floating," composed of clean sand and cement in 

 proportions of 3 to i, is also mixed by a light portable 

 mixing machine. 



The wood paving blocks are usually specified to be 

 of the best Swedish yellow deal and to be cut from 

 3 by 9 in. planks. They are 8 in. or 9 in. long, 3 in. 

 wide, and 4 to 5 in. deep. The blocks are placed in a 

 cylinder, which is raised to a temperature of 200 degrees 

 Fahrenheit. The steam is exhausted from the cylinder 

 by means of an air pump, and creosote is forced into 

 the blocks at a pressure of not less than 120 lb. to the 

 square inch and to an amount of not less than 8 lb. 

 to each cubic foot of timber. The blocks when laid 

 are thoroughly " grouted' with pitch and creosote oil 

 mixed in proportions of 8 to I and then washed over 

 with liquid cement. Finally they are covered with 

 I in. of crushed shingle, and the road is then ready 

 to be reopened for traffic. 



Even with the introduction of machinery wood 

 paving renewals are a costly business, and the esti- 



