As the pendulums were first drawn to the same side through the same angle, they will make their oscillations also in such a way that at every moment their inclination will be the same (i.e. the oscillations will occur with identical phases). Then the spring will not be extended and, if it is weightless, will have no effect on the pendulums' period of oscillation: each pendulum oscillates with a period which belongs to it separately. The mid-point of the spring also makes oscillations in this case. But if the spring be fixed at its mid-point, the spring will begin to affect the pendulum's period of oscillation, since it will be extended and compressed during oscillation. The pendulum's oscillations will take place not only under the influence of its own weight, but also under the influence of the spring's elasticity, which always acts with a force directed towards the position of equilibrium, and therefore increases the pendulum's acceleration by comparison with that which it would have under the influence of only the force of gravity. This leads to a reduction of the period of oscillation.