This is a system of more than two gears, connected together. The rotary motion of the first wheel drives the second wheel, and so on. In this system, the rotary movement of the first axle is transmitted to the second by means of wheels 1 and 2.
Wheel 3 rotates with the same velocity as wheel 2 and drives wheel 4, to which it's connected.
Each connected geared wheel rotates in the opposite firection to the wheel it's attached to.
N4/N1=(Z1 x Z3) / (Z2 x Z4)
N1 and N4 are the velociies of the frive wheel and the driven wheel
Z1,Z2,Z3 and Z4 are the number of teeth on the wheels