I have always loved the curves of old kerosene lanterns.  These two belonged to my grandmother and came to me when she died.  The reason they are shaped so curvaceously is because the glass chimney needs a “throat” or slight constriction to make a draft.  The draft carries oxygen past the flame producing a brighter, smokeless light than an open flame can produce.  There endeth the science component of the post.. I think the curves of the glass just look elegant..