42' APPLICATION OF THE LAWS OF HYDRAULICS
by the Yesmann formula. For descending roofs, it will be greater.
In his work, Professor Yesmann has given the mathematical
expression for the first case, but he has introduced certain coeffi-
cients of friction for which no accurate data exist at present.
In what measure is actual usage found to accord with this
formula. For verifying Yesmann's formula, a test furnace has
been built at the Polytechnic Institute of Petrograd; but during
the time that it has been in service it has been impossible to
obtain a gaseous current with a lower free surface or nappe which
Scale A =
FIG. 26.—Values of Coefficient A.
was free from eddies. In this current of gases the surface is mixing
with air. For this reason the measurement of the thickness of the
gaseous stream is difficult. It is hoped that this difficulty may be
overcome, and that it will be possible to effect the verification of the
formula for the flow of gases under inverted weirs by accurate
methods. In the meantime, Yesmann's formula has been tested
by application to furnaces which were in use. The tests have been
based upon the assumption that since the furnace works well and
the shop is satisfied, the hot gases which flow in the inverted chan-
nel must lick the hearth of the furnace in a satisfactory manner.
The reverberatory furnace itself forms a true inverted channel