44 
Rural Roads. 
macadam which have led to a great variety in the results 
hitherto obtained : first, the necessity for dry weather during 
the execution of the work ; secondly, the varying qualities of 
tar, which is a highly complex substance. If over-distilled it 
loses its binding qualities, and if under-distilled it softens in 
warm weather. Moreover much of the tar produced with modern 
systems of gas making, is not of a kind well adapted for tar 
macadam ; and on the whole, though this kind of treatment 
has given most satisfactory results where skilfully applied 
in good weather, it requires very careful experimental in- 
vestigations such as are now being carried out by the Roads 
Improvement Association, before it can be considered quite 
satisfactory or reliable. 
The life of tar macadam is about twice that of ordinary 
macadam and it requires to be re-surfaced in two to ten years 
according to the traffic. Tar-mac laid at Newark showed no sign 
of wear after being subjected to very heavy traffic for two years. 
Mr. Aitken, of Cupar, Fife, a most successful highway 
engineer, and the author of one of the most modern books 
on roads, has lately invented a tar-spraying machine, using 
compressed air, with which he hopes to be able to prepare 
tar macadam, and to apply tar to the surface of finished roads 
at a trifling cost. Mr. A. G-ladwell, Surveyor to the Eton Rural 
District Council, lately reported that he is obtaining excellent 
results in re-coating roads by rolling clean granite into a bed 
of fine stone chippings, about three-quarters of an inch thick, 
mixed with tar. He states that under the roller the tarred 
mixture is forced upwards, thus filling the joints in the granite 
coating in a satisfactory manner, and that owing to the ease 
and rapidity with which the work can be done, and the saving 
of watering when rolling, no extra cost is involved in laying 
the improved coating. Many experiments have been made to 
secure a thin waterproof and durable skin or coat on the top 
of macadam, but the prospects of success in this direction 
appear to the author to be slight. 
What engineers are in. need of is a material of moderate cost, 
better than tar, for use as a binder or matrix for macadam ; and 
the writer believes that if chemists experimented with Trinidad 
bitumen and other similar substances, what is required might 
soon be found. Mr. Brodie, the City Engineer of Liverpool, has 
already attained a considerable degree of success in this direc- 
tion. The use of Portland cement as a binding material was 
tried many years ago, and it appears to be worthy of new trials ; 
but obviously with such a rigid binder it would be necessary 
to have a very rigid foundation. It is stated that concrete 
pavements for heavy traffic have been thoroughly tested with 
good results in Chicago and Denver during the last two years. 
