\ X 3 ^2 



Figure 27-3. 



If x 2 — %! is small, the change in log (u 2 + z 2 2 ) and log {u 2 + Zt 2 ) 

 along the interval will be approximately linear and we may use their values 

 at the center of the interval so that approximately 



A' e = k P [ log (x,* + z,») - log (x s 2 + z 2 ) ] (x 2 - x t ) (20) 



To use this result to find the value of A z for cylinders having any cross 

 section, we first divide the area into narrow vertical rectangles as shown in 

 Figure 27-4, evaluate the right hand side of (20) for each rectangle, and 

 add the results. In practice this would be done by adding the values of 



Figure 27-4. 



590 



