16 



BITLLETIlSr 190, U. S. DEPARTMENT OF AGEICULTUEE. 



cleaning device, operated by sewer rods. If it is expected that such 

 work will be necessary the drains should be laid out in straight lines, 

 with grades as uniform as possible; and a manhole should be pro- 

 vided at each junction, change in direction, change in slope from a 

 steep to a lighter grade, and on straight sections at intervals not 

 exceeding 500 feet. 



On straight lines a manhole may be made long and narrow, but at 

 a junction or turn it should be made square to facilitate the operation 



of rods and to pre- 

 vent the sediment from 

 being carried across the 

 trap. A convenient 

 size is 4 feet square. In 

 constructing the man- 

 hole, brick or concrete 

 is preferable, but lum- 

 ber is often employed. 

 When built of lumber, 

 2-inch material should 

 be used. Figure 8 

 shows a simple type 

 of manhole, constructed 

 of lumber and so de- 

 signed that earth pres- 

 sure will maintain it in 

 spite of the failure of 

 the nails. Figure 18, 

 page 23, illustrates the 

 application of man- 

 holes. In most places 

 the manhole should 

 be provided with a 

 bottom and it should 



Fig. 8. — Combination manhole and sand trap witli cover (Miller 

 design), that does not depend upon naUs to hold it together. 



always be fitted with a cover that may be locked down. 



OBSERVATION WELLS. 



If little sand be present, rendering sand traps unnecessary, it is 

 still desirable that opportunity be afforded for observation of the 

 flow at various points throughout the system. Nothing serves this 

 purpose better than a vertical stack of large-sized tile, extending 

 from a little above the surface of the ground to a foot or more below 

 the tile line, and having holes cut in the lower length to accommodate 

 the drain. Such a device is shown in figure 9. As may be seen, a 

 smaU settling space is provided from which sediment may be removed 

 from time to time by means of a telephone spoon. A cover should 



