206 EARTHWORK 



between Sta. 24 and Sta. 24+35, there should be obtained, 

 provided the prismoid is 100 ft. long, 1,132+684-297= 1.519 

 cu. yd. As the length is but 35 ft., the actual value of V a 

 is -ffoX 1,519 = 532 cu. yd., which is written in column 7 (&). 



It is usually more convenient to compute all the numbers 

 in each column before passing on to the next column. When 

 column 7 (b) has been filled up, the number in this column 

 opposite each station is the approximate number of cubic 

 yards, computed by average end areas, contained between 

 that station and the preceding station. Thus, 1,048 is the 

 approximate number of cubic yards between Sta. 23 and 

 Sta. 22; 531 is the approximate number between Sta. 24+35 

 and Sta. 24; etc. The total approximate number of cubic 

 yards, between Sta. 22 and Sta. 25, as computed by average 

 end areas, is, therefore, 1,049 + 1.602+532+426 = 3,609 cu. yd. 



The prismoidal correction must now be computed. 



Since the result is to be expressed in cubic yards, the pre- 

 ceding formula for C becomes 



C= - (w-w 1 ) (d'-d) 

 12X27 



The successive values of w w' in column 8 are obtained 

 by subtracting each number in column 6 from the number 

 just below it in this column. Thus, for the prismoid between 

 Sta. 22 and Sta. 23, w = 46.3, / = 49.6; and w-w f = -3.3 ft. 

 Similarly, the values of d' d in column 9 are obtained by 

 subtracting each number in column 2 from the number just 

 above it in this column. Thus, for the first prismoid, d = 6.2, 

 d' = 9.4, and d'-d= +3.2 ft. 



The numbers in column 10 are the computed values of the 

 prismoidal correction C. Thus, for the first prismoid, since 

 /=100, 



c=~x -3.3X3.2= -3 cu. yd. 



for the second prismoid, 

 100 



and similarly for the remaining prismoids. 



The volume of the first prismoid, as obtained by the 



