952 
Journal of Agricultural Research 
Vol. V, No. 20 
This consideration has led the Office of Public Roads and Rural Engi¬ 
neering to attempt to gain definite knowledge of the wear of concrete 
roads carrying various kinds of traffic, and a special instrument has been 
designed by the writer and built in this office for that purpose. Several 
methods of taking autographic records of the cross section of the road 
were considered, but were discarded in favor of the simpler and more 
portable form of instrument finally constructed. 
Essentially, this instrument consists of a fine wire stretched tightly 
across the road at a constant height, together with an inside micrometer 
for measuring the distance from the road surface to the wire. Measure¬ 
ments taken i foot apart across the road permit the plotting of its cross 
section, and if these measurements are repeated at long intervals the 
change of cross section or the decrease in the thickness of the road will 
be revealed. 
The accompanying illustrations show the instrument in detail and its 
method of application on the road. If Plate EXVI, figure i, and text 
figure i are first referred to, the component parts of the apparatus may 
be seen very plainly. 
Pieces A and B are made of cement mortar and have embedded in them 
steel rods, C, drilled with holes slightly inclined with the horizontal. A 
fine piano wire about o.oi of an inch in diameter is passed through these 
holes and is stretched across the road from block A to block B . The tops 
of these rods are each provided with a disk-level bubble, so that when 
placed in position in the road the rods may be adjusted to a vertical 
position. Block A, which is heavier than block B, is provided with two 
adjusting screws, D, for adjusting rod C to the vertical. Block B rests on 
two points only, one the lower end of rod C and the other the end of 
adjusting screw D . Constant tension is produced in the wire by the 
weight of block B, which is pivoted about the bottom of rod C and is 
adjusted to a horizontal position by means of rack E , provided at the end 
of the wire. As the weight of block B is constant, the tension in the wire, 
and consequently the amount of sag for like spans, must remain the same. 
A very definite and fixed datum is thus provided, which should remain 
constant from year to year and which is very easily established by merely 
placing the end blocks of the apparatus in their proper position on the 
road. 
The bottoms of rods C are spherical in shape; and when in use on 
concrete roads, they rest on the flat tops of bronze plugs cemented in the 
road surface. These plugs are % inch in diameter and are i ^ inches 
long. They are set % inch below the surface, and their tops are protected 
by means of a brass pipe plugged with a bituminous-sand mixture during 
the long intervals between readings. 
In obtaining the wear measurements a chalk line is first snapped 
across the road between the bronze plugs, and the points at which it is 
