I have often seen people ask how to wire multiple speakers to their amp to get a higher output power. This is achieved by decreasing the impedance of the load to get the highest output power the amp will provide, as shown by the formula P = I²R where P is power in Watts, I is current in Amps and R is resistance in Ohms. It is important to remember that Impedance and Resistance are not the same thing, as Impedance takes into account the effects of resistance, inductance and frequency. However, in this case it is safe to assume that resistance and nominal Impedance are the same thing.
Wiring speakers in series increases the overall resistance of the system. In a series circuit, the speakers are wired from output to input in a chain, where the same current is shared between all speakers in the circuit.