Figure 27-8. Contour map of log (x + V x2 + y 2 + D 



be carried out, one for the semi-infinite prism whose top is 100 feet below the 

 surface and one for that whose top is 300 feet below the surface. Suppose the 

 long edge of the body to run east and west and that the point where we wish 

 to compute A g is 200 feet west and 100 feet south of the southwest corner of 

 the body. Then, the positive direction of the x axis being taken north, the south- 

 west corner of the shallower body would be placed at the point x = 2, y — 1 and 

 that for the deeper body at the point x = 2/3, y = 1/3. In units of the contour 

 map the shallower body would measure 3 units by 2 units while the deeper 

 body would measure 1 unit by 2/3 unit. 



Figure 27-8 shows the contour map with the superposed outlines of the 

 two bodies. The values of log Cx + \^x 2 + y 2 + 1 ) have been estimated at the 



595 



