16 BULLETIN 556, U. S. DEPARTMENT OF AGBICULTUBE. 
to falling bodies or to other conditions in which the stress is applied 
and relieved in one-twenty-fifth, of a second or less. It represents a 
quality important in tool handles and in athletic goods, such as base- 
ball bats. 
HEIGHT OF DROP. 
Height of drop is the maximum or last drop of the hammer. It 
represents a quality important in articles which are occasionally 
stressed under a shock beyond their elastic limit, such as handles and 
vehicle and implement parts. 
COMPRESSION PARALLEL TO GRAIN. 
In the compression parallel to grain test a 2 by 2 by 8 inch block is 
compressed in the direction of its length. Deformation is measured 
between two collars attached 6 inches apart to the specimen. 
FIBER STRESS AT ELASTIC LIMIT. 
Fiber stress at elastic limit in compression parallel to the grain is 
not much used because in most cases it is more convenient to use 
maximum crushing strength, which is less variable and easier to 
obtain. The value is important in the derivation of safe working 
stresses for structural timber. (See also fiber stress at elastic limit, 
glossary, p. 21.) 
MAXIMUM CRUSHING STRENGTH. 
The maximum crashing strength is the maximum ability of a short 
block to sustain a slowly applied load. It is obtained by dividing 
the maximum load obtained in the test by the area of cross section 
of the block. This property is important in estimating the strength 
of columns. 
Tests of the crushing strength, because of their simplicity, are 
frequently the only tests used in studying the effect of various 
influences or processes on strength. Crushing strength is not neces- 
sarily representative of the other strength properties; consequently, 
when used alone, it will occasionally lead to erroneous conclusions. 
For instance, it was found that the crushing strength of some timbers 
was increased 10 per cent by a certain heat treatment. Other tests, 
however, revealed the fact that their resistance to shock had been 
reduced about 50 per cent. 
A safe working stress for carefully selected structural timbers used 
as columns and in dry interior construction, all exceptionally light 
pieces excluded, is about one-third the crushing strength as given 
in the table for tests on green materials (Table 1). If the column is 
longer than about 10 times its least diameter, some formula should 
be used which will take care of the increased stress which would be 
caused by eccentric loading or by the bending of the column. (Such 
