WwooD 61 
boards would have curled up or warped, as shown in 
Fig. 76. 
Besides warping, the evaporation of the sap causes 
the whole tree to shrink in diameter, and consequently 
our planks will tend to become narrower. This is 
called shrinkage, and in some woods amounts to a 
quarter of an inch to the foot, which means that a 
plank sawed twelve inches wide will, after a few 
months, measure only eleven and 
three quarter inches. BEES 
When we construct anything in 
wood we must always consider how 
the object will be affected by warp- CWeaLZzy 
ing and shrinkage, remembering WE? 
that the shrinkage is only across yy. 76. showing Effect 
the grain. of Warping 
Let us consider the problem of constructing a draw- 
ing board to see how warping and shrinkage may be 
Sm 
COT TK ff 
overcome. 
If we make it of one piece, like A (Fig. 77), the board 
will soon change its shape to that shown in B, which 
would make it useless for mechanical drawing, as a per- 
fectly flat surface is necessary. We can overcome the 
warping by screwing heavy cleats on one side across 
the grain, as shown at C. The cleats would need to be 
heavy or the warping force would bend them. 
