Complex sentence: The gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726, which didn't change in length with temperature, so that its period of swing stayed constant with changes in ambient temperature.
Split into simpler sentences: The gridiron pendulum was an improved clock pendulum invented by British clockmaker John Harrison around 1726. It didn't change its effective length with temperature, so its period of swing stayed constant with changes in ambient temperature.