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                                      158                                    APPENDIX I

I'i '

from which

If the thickness of the crest of the inverted weir exceeds 1 m,
the resistance to the flow of the gas will be increased and it will

                                     be necessary to increase the value of h.    On the contrary, if the

thickness of the crest and the roughness of the wall surface are
p                                     diminished, the coefficient in the last formula will become less,

ij J'                                   and its minimum value corresponding to M = 1 will, as it is easy to

t<;                                   see, be equal to or very close to 3.05.

|! I                                    The preceding formulas have been established upon the sup-

11 |                               position that the gas arriving at the bridge wall has a horizontal

\\                                 velocity equal to zero.    But it follows from these considerations

11 \                               that, in the case where the horizontal velocity is not a negligible

'" *                                 factor (Fig. 132) the form of the formula does not change.



1    i

i         1

FIG. 122.
In these cases it is evidently necessary to increase the value of
the coefficient H to H-0.55, conforming to^Bazin's formula,
p being the depth or thickness of the flowing current of gas before it
reaches the sill or weir, and being equal to the height of the stream
over the weir plus the value of H. When this last value is known,
it becomes possible to introduce into the formula a correction for
each particular case, as well as in the preceding computations.
It will be seen in the numerical examples examined that -the
relation between the thickness or depth of the gaseous stream which
flows through the opening over the bridge or its depth below the
inverted weir, the volume of gas per second Q and the length of
the crest of the weir at right angles to the direction of flow B, is
finally given by the equation: