The idea of turning is applied to the fire. Typically one would not think of the flames turning. But the speaker is applying the turning motion here to keep the motion of turning. The speaker is bringing in all the symbols of the poem and applying them to the idea of time passing. The flames of the fire climb upward and then burn out. Fire naturally has an upward motion as opposed to the turning motion the speaker has given it. Like all things in life, the fire will eventually burn out. The fire also brings the audience back to where the speaker is, in the room by the fire. The fire is the only thing at this point that is stable. The leaves and the peacocks seems to be just images of memory, but the fire is not a memory.