as possible, by reducing their velocity. This effect is obtained
under the crest of an inverted weir for gases, in the same manner
in which it would be done on a weir carrying water, by providing
a cistern or cutting a deeper reservoir in the weir, making an
upstream and a downstream crest.
When a weir having a thick crest (Fig. 34) is transformed into
a weir with a cistern or reservoir (Fig. 35), the velocity of flow of
v\S ^...-^ _..-.^- ^—*^_
n n n
the water in the reservoir is reduced, while the level of its free
surface remains the same.
In securing this result with an inverted weir for gases, all of the