38 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
then likely to occur. In such cases it may be advantageous to cut 
the section into a larger number of prongs, thus reducing the degree 
of curvature in each and hence permitting surer comparison of the 
relative lengths of the individual ones. If they are thin enough 
there will be but little difference in stress between the opposite sides 
of each prong, and its state of stress will consequently be correctly 
indicated by the change in its length. , 
All stress-section prongs in tension at the time of cutting will 
shorten, and those in compression will relieve themselves by length- 
ening. The outer ends of such prongs will form a curve, as shown 
roughly by sections 2 and 4 of Figure 8, and the shape of this curve 
will indicate clearly the state of stress. If it is convex or high in 
the center, as in section 2, it denotes tension in the outer layers and 
compression in the core. If low in the center, it means the reverse. 
So far only general indications at the time of sawing have been 
considered. If the sections are now set aside in a suitable place they 
will soon dry down to an approximately uniform moisture content, 
the actual value depending upon the temperature and the humidity 
of the surrounding atmosphere, and these last changes in the moisture 
content of the section will be made evident by further changes in the 
length and in the curvature of the individual prongs. Loss of mois- 
ture on one side of a thick preng will usually be most plainly shown 
by a change of shape, the prong bending toward the side that has 
just been drying. If, however, there is an equal loss of moisture from 
both its sides, which may happen if it is in the middle of a section, 
the only indication will be a shrinkage in its length. In sections 
taken from thin lumber this is apt to be the fact anyway, because the 
very thinness precludes much difference in moisture content between 
the two sides of the board and allows still less between the sides of a 
prong. : 
Under ordinary circumstances, the final drying of a stress section 
will cause a contraction or an inward turning, or both, to take place 
in all the prongs, the amount in each prong being dependent upon 
the amount of moisture lost from the individual prong. Except 
when steaming or other conditioning treatment is given the stock in 
the kiln after the stress section has been taken out, the final shape of 
the entire section is a criterion by which to judge what the condition 
of the stock will be after the drying has been completed. Caution 
must be used in judging, however, since the sections dry without 
further stress and the stock in the kiln probably does not. The more 
nearly dry the stock is when the stress section is cut, the more reliable 
an indicator will it be in this respect. 
The following key interprets the significance of the behavior of 
the stress sections when they are first cut and then after room drying. 
In using this key and in comparing various sections one with another 
it must be remembered that the thickness of the stock and the width 
and the number of the prongs have an important bearing upon the 
appearance and the behavior of the individual sections. 
