Revelation 17:10
'They are also seven kings. Five have fallen, one is, the other has not yet come; but when he does come, he must remain for only a little while' - the seven heads are also seven kings or kingdoms: five are already fallen (past empires), one is reigning (Rome in John's time), and one is yet to come (a future empire). The succession of empires shows the transient nature of human power.