6 BULLETIX 373, U. S. DEPABTMEXT OF AGBICULTTTEE. 
or not a given quality of brick is suitable for use in constructing 
pavements and to furnish a basis for comparing different classes of 
brick. The methods have, therefore, been repeatedly changed, not 
only in order to make the results obtained indicate more definitely 
the quality of the brick, but also with a view to establishing uniform- 
ity, so that results obtained in different laboratories may be intelli- 
gently compared. A discussion of the most important tests follows 
in more or less detail. 
FIELD TEST. 
The general appearance of a paving brick is. to an experienced 
eye. a valuable indication of its quality and will frequently suggest 
the advisability of applying routine tests to some particular part of 
a shipment. Unfortunately the knowledge gained from experience 
with one kind of brick can not be safely relied upon in inspecting 
other brick made by a different process or from a different class of 
raw material. A further limitation to this method of testing lies 
in the fact that the results obtained do not admit of numerical evalu- 
ation, and can not. therefore, be very accurately described. This 
test is nevertheless valuable, and since no apparatus other than a 
hand hammer is needed, it can always be employed. 
The test consists simply in making a careful inspection of the 
brick individually and collectively. The size is tested by making 
measurements, the shape by arranging a number of brick in the order 
in which they are intended to be placed, and the quality by an exam- 
ination of both the exterior and interior of a number of samples. 
TRANSVERSE TEST. 
The transverse strength of a brick is determined by supporting it 
upon two knife edges and applying a load on the opposite side and 
midway between the supports by means of a third knife edge. The 
load is gradually increased until rupture occurs, and the result of 
. . 3PZ 
the test is expressed in terms of the ratio 7yTj2 called the modulus 
of rupture. In the above ratio P represents the breaking load in 
pounds, while 7. 5, and d represent, respectively, the distance between 
supports, the breadth of the specimen, and the depth of the speci- 
men, all measured in inches. 
The modulus of rupture for good paving brick usually lies between 
2.000 and 3.000 pounds per square inch, and frequently varies con- 
siderably even with carefully selected specimens which have been 
manufactured under identical conditions. 
RATTLER OR ABRASION TEST. 
The rattler or abrasion test is undoubtedly the most important of 
the tests made on paving brick at present. In making this test the 
