It was apparent to John Harrison that the greatest error to be counteracted in pendulum clocks fitted with a good escapement was that due to expansion or contraction of the pendulum rod in heat or cold. The Harrison brothers measured the coefficients of expansion of most metals and alloys available then and discovered that brass expanded relative to steel in the ratio approximately of 10 to 6. They made pendulums working on this principle, incorporating four brass rods acting upwards, and five steel rods acting downwards. The upwards expansion of the brass rods compensated for the downwards expansion of the steel rods and maintianed the centre of oscillation of the pendulum at a constant distance from the suspension point. This principle was used in their early marine chronometers to alter the effective working length of the balance spring, thus compensating for their temperature errors.