an external one which passes under the chimney-holder externally to the
wick. In giving the upper part of the chimney, properly so called, the
form of a truncated cone whose smaller base is turned toward the internal
current of air, that is to say, in directing this current toward the
contracted part of the upper cone, at the point where the depression is
greatest, a strong suction is brought about, which has the effect of
carrying along the air between the wick and glass, and giving it its own
velocity. The draught of the two currents having been effected through the
conical form of the upper part of the chimney, it remained to regulate the
entrance of the external current into the flame. If this current should
enter the latter at too sharp an angle, it would carry it toward the mouth
of the chimney before the chemical combustion of the carbon and oxygen was
finished; and if, on the contrary, it should traverse it at too obtuse an
angle, it would depress and contract it. Experience has shown that in the
majority of cases the most favorable angle at which the external current
of air can be led into the flame varies between 35° and 45°. We say in the
majority of cases, for there are exceptions; this depends upon the
combustive materials and upon the conditions under which they enter the
flame. The annexed figure shows the form adopted by the inventor for oil
and kerosene lamps. As may be seen, the chimney consists of two cones, A
and B, connected end to end by their small bases. The upper one, A, or
divergent cone, is constructed according to a variable angle, but one
which, in order to produce its maximum effect, ought not to differ much
from 5°. This cone rests upon the convergent one, B, whose angle, as we
have said, varies between 35° and 45°. To the large base of this cone
there is soldered a cylindrical part, c, designed for fixing the chimney
to the holder. The height given the divergent cone is likewise variable,
but a very beautiful light is obtained, when it is equal to six times the
diameter of the contracted part. When the lamp is designed to be used in a
still atmosphere, free from abrupt currents of air, the height may be