Results of the Experiments. 
253 
what would be expected, their values being 54.5 and 53.9. 
They prove that with stones like granite, which have a low 
cementing power, a good quality of limestone may be of con¬ 
siderable value as a binder. The diabase of number 5 with a 
value of 62.1, the limestone of number 6 with a value of 58.8, 
and the fine granite of number 7 with a value of 55.9 show 
nearly the same value as binders; but in actual practice lime¬ 
stone is much inferior to the others because of its low coefficient 
of wear, which is less than half that of diabase. Being rapidly 
reduced to a powder, it is washed away by heavy rains or blown 
away as a disagreeable dust. On this account roads on which 
it is used require constant repairs and its first cheapness is 
offset by this later expense. Numbers 1 to 4 are rather a sur¬ 
prise since they indicate that the cementing power of mixtures 
is higher than that of stone of a single kind. The cements and 
values of these four are as follows: diabase and limestone 100, 
diabase and sand 78.5, fine granite and limestone 62.4, fine 
granite and sand 62.3. In actual practice, if diabase were used 
as a binder for the upper part of a macadam road made of gravel, 
the cementing value would be 85 or 90 and if granite were used 
the value would be about 62. This means that a road whose 
surface was half gravel and half diabase would oppose twice as 
much resistance to the breaking action of horses’ feet as would 
one made wholly of gravel. 
The results of the experiments which have been described 
above are merely preliminary to future work and rest upon too 
small a body of data to be taken as final. Future and more ac¬ 
curate tests may and undoubtedly will give results which differ 
widely from those here given. There is urgent need for ex¬ 
tended investigations along this line. Many towns are begin¬ 
ning to build macadam roads and if it be true, for instance, as 
these experiments indicate, that by spending fifty per cent, more 
for the transportation of basic rocks like diabase for use in con¬ 
nection with drift material in building the upper parts of the 
macadam, a road can be built which will last twice as long as 
one made of pure gravel, it is time the fact were known. The 
cost of building one mile of gravel macadam road fifteen feet 
wide and one foot thick is about $3,500 under favorable condi- 
