Drainage in New York. 261 



ment is shown in Fig. 198 and in Fig. 199 is shown the use of a rope 



or cord to secure a good curve for the union of a 

 right angle lateral with the main in making the 

 first cut in the ditch. 

 Fig. ig8.— C r r e ct The grade stakes should be set from 12 to 18 inches 

 twTlUics of tile. to one side of the center of the ditch so that they will 

 not be disturbed during the excavation, 

 (i) Fall. — All lines of tile should run with the greatest slope. Even 

 on very rough land the drains should be run directly instead of diagonally 

 down the slope. The fall should be as great as the condition will permit. 

 The greater the fall, the greater will be the carrying capacity of the 

 ditch. It is a rough general rule that doubling the fall increases by 

 one third the carrying capacity of a given size of tile. It is very desir- 

 able to have a fall of at least four inches per 100 feet, but drains have 

 operated well with a much less fall — even as small as one inch in 100 

 feet and under special conditions, where the 

 water flows under a head developed at some 

 other place, tile have been laid on a dead 

 level. Where the fall is less than ten inches 

 per 100 feet, it is desirable to use an in- 

 strument in laying out the ditch and if it "*(:TWiVy-'Cr';&*''v-; 



:•.>'■•= 



is less than three inches, the greatest care of *IV^I/f^/¥'S>c'i:^f^4r^ 



an experienced engineer should be employed. ^^|^:.;^My\<';|;.;-;/>^'.^''^ 



Fortunately there are few situations where the \^]^wM '^''"''" 

 available fall is so small as to make such 



refinement necessary although it is frequently 



.1 ,, 1 r ii i-i. 1 Fig. 199. — Shozcs use of 



necessary to have the upper end of the ditch ^^rrf to form a smooth 



much more shallow than the lower end to de- cMrt'<? in joining lateral 



, ,1 r 11 zvith mam ditch. 



velop the fall. 



In the distribution of the fall where it is necessarily different in 

 different parts of the system, it should be the aim to have the 

 greatest fall nearest the mouth. A lateral drain should never empty 

 into a main drain having a less fall. The most rapid flow will corres- 

 pond to the greatest fall and particles of soil which would be carried 

 in the lateral would be deposited in the slower flow of the main, 

 permitting it to clog. However, this rule does not oppose the use of 

 a large fall of the last rod or two of the lateral to bring it into the 

 main at the same level. Several types of union between laterals and 

 the main are shown in Figs. 200, 201 and 202. The latter illustrates the 

 best and probably the most used type. A few stones may be placed 

 around the joint to check any tendency to the entrance of soil, which 



