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 deg. and 45 deg.. 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.
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