16 
BULLETIN 190, U. S. DEPARTMENT OF AGRICULTURE. 
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 with cover (Miller 
design), that does not depend upon nails 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 
small settling space is provided from which sediment may be removed 
from time to time by means of a telephone spoon. A cover should 
